I have been thinking a lot about the ways in which Avalanche has changed through the various iterations of this story.
1. OG Avalanche
If I recall correctly (and I make no claims to having an infallible memory) the Avalanche of the OG was a pretty small group. We only see Tifa, Barret, Biggs, Wedge, and Jessie. However, other operatives are referred to:
Think how many of our people risked their lives, just for this code...
The Avalanche propaganda around town shows that their reach is long even if their group is small.
That said, my personal impression of Avalanche's activities in the OG was that they were doomed to fail. They were a tiny gnat beating themselves senseless against the thick hide of a behemoth, driven on as much by rage and a thirst for revenge as by the more objective goal of saving the planet. Shinra was just too big and too powerful for them to have any real impact. The sheer futility of their efforts gave them a real poignancy, but it also highlighted their moral ambiguity. All those people killed in the Reactor 1 bombing died for nothing, just like all the people killed in 9/11. Avalanche can't win.
In the OG, the plate drop was out of all proportion to the threat Avalanche posed. That was the point of it. Shinra were using a rocket launcher to take out a mosquito.
Despite the fact that two of its most beloved characters were terrorists, the OG did not endorse terrorism. Barret and Tifa were shown to have dedicated their lives to a futile cause, and even though its goal was admirable, the means they chose - which cost many innocent lives - were not justified by the end, particularly since the end was neither realistic or achievable.
2. BC Avalanche
Disclaimer: I spent so long writing about this game and trying to tidy up its glaring plot-holes and absurdities that I am pretty sure I don't always remember it correctly.
That said,
At Cosmo Canyon in the OG Barret gives us the potted history of Avalanche:
There was this guy who studied 'Planetary Life' here.
He couldn't take things the way they were, so he went to Midgar to form AVALANCHE.
Wanna hear more?
Cloud
- Go on
Barret
Guess you could say this is where AVALANCHE was born.
The writers of
Before Crisis took these few lines and built on them to create an Avalanche that was a primary antagonist for the Turks, and as evil in its own way as Shinra. The man to whom Barret refers became Fuhito. Fuhito, your typical mad scientist and Hojo fanboy, might have started off seeking to take down Shinra, but his goals eventually morphed into destroying the entire world, "cleansing" it of corruption, by using the summon materia embedded in the hand of Elfe, a warrior capable of going toe to toe with Sephiroth and living to tell the tale. Lo and behold, Elfe turns out to be none other than the long-lost daughter of the leader of the Turks, Veld (or Verdot), and the Turks make the entirely rational and plausible decision (sarcastic cough) to risk the destruction of the entire world in an attempt to save her.
In one of the game's big, and well known, plot twists, it is revealed that Rufus Shinra has been providing Fuhito with funding. Exactly how he managed to do this without ever being found out, or indeed how he made contact with Avalanche in the first place, is left to our own imaginations. In return, Avalanche was supposed to kill Rufus's father, something which they continually fail to do. Was this failure deliberate? You decide. The Turks seemed very slowly to be winning the war against Avalanche, destroying its bases, trimming its size. Eventually Fuhito turns on Rufus, Rufus is arrested, the Avalanche leadership turn on each other, and there's a time skip of, I think, 4 years, during which time nothing is heard of them.
It would be boring to re-tell the whole story. The main points I wanted to make are these:
1. BC Avalanche is a much bigger threat to Shinra than OG Avalanche. Fuhito engages in human experimentation
à la mode de Hojo, squanders human lives as freely as Shinra, and ultimately poses an even bigger and more immediate danger to the planet. This is why Shalua Rui, a "good" character, left them.
2. Nowhere in BC is it suggested that there are different wings or factions in Avalanche, of which Fuhito's is just one, and the lunatic fringe at that. Nowhere is it suggested that Rufus wasn't funding the entire operation. Nevertheless, there is plenty of space left 'between the lines' to read that interpretation into it, which means that nothing said in the Remake about Avalanche explicitly contradicts anything in BC, even if it doesn't exactly align with it.
Also, I find the suggestion that Rufus was blindly funding the "wrong" wing of Avalanche, and being taken for a financial ride by Fuhito, quite hilarious. I think that might become my new headcanon. I'm sure Rufus minds being scammed much more than he minds attempted murder.
3. If Fuhito wasn't the man who left Cosmo Canyon to start Avalanche, then who was? Perhaps some as yet to be revealed new character?
4. The Barret we first meet in Midgar has never been to Cosmo Canyon. We can infer that he found out about and joined the resistance movement when he arrived in Midgar, although he could have learned about Avalanche anywhere on his travels. He might even have come to Midgar specifically for the purpose of joining them. Maybe the Ultimania will tell us more, maybe it won't.
I think it goes without saying that Barret would never have joined an organisation that had Fuhito's goals and used Fuhito's methods; or if he did join, that he would swiftly back away and do his own thing. While Barret is able to tolerate a certain amount of collateral damage (using the doctrine of double effect), he would draw the line a long way before he got to human experimentation.
tl;dr BC Avalanche is a much bigger threat to Shinra than OG Avalanche. It's also a huge threat to the planet. But due to the nature of the media, the threat it poses is cartoony: a giant Godzilla summons almost destroys the entire planet, and the Turks have to go into a singularity to defeat it. BC is a pretty silly story when all is said and done.
3. Remake Avalanche
Remake Avalanche is a big, BIG organisation. It has separate wings with multiple cells. It has its own helicopters. (In BC, they had to steal one from Shinra). Its structure and leadership isn't clearly explained in the game, and perhaps it is in truth pretty incoherent, with different leaders leading different wings who are constantly at odds with each other (like the liberation armies of Africa).
Barret's cell seems to be some kind of splinter organisation which is cut off from the core, though it is unclear why. Because their methods are too violent? Because their methods aren't violent enough? Whenever we see "main" Avalanche in action, they seem to be every bit as violent as Barret's cell, and much better equipped. They mow down Shinra's troops in Chapter 4. They plant a bomb that rocks the Shinra Building in chapter 17. They have sufficient control over the building to send a helicopter to the roof to pick up Barret and his gang and reasonably hope that this rescue will be a success.
In short, we can make all kinds of educated guesses as to why Barret fell out with Avalanche HQ, but we don't really know. It can't be because he opposed Fuhito's goals and methods. Fuhito is never mentioned as far as I can recall. We don't know yet if Fuhito is even canon in this continuity, but if he is, his own wing is now nothing more than a splinter cell.
Unlike in the OG, where Barret's little Avalanche was portrayed as beating its puny fists fruitlessly against a giant steel wall, Remake Avalanche has the manpower and resources to pose a real, serious threat to Shinra. They literally rock Shinra to its foundations. Their terrorism is portrayed as having less consequences for innocent people than OG Avalanche's was (because now the real damage is all Shinra's fault), and therefore it is more justified. In turn, the plate drop, while still horrific (but with many more survivors) becomes just another escalation of the ongoing war of attrition between two more evenly matched adversaries, Avalanche and Shinra.
In short
The implication in the OG was that Avalanche was a tiny organisation, albeit bigger than the five characters we saw, and that their struggle against Shinra was doomed to failure. In the Remake, Avalanche is a huge organisation capable of posing a real threat to Shinra's continued existence.
Please note, I'm not trying to argue here that the Remake is wrong, or anything like that. I'm only trying to catalogue the changes Avalanche has gone through. 
