Probably a sliding scale.
Start at low-key upgrades and unlocks, but once a faction's gotten a step or two into the tech tree, then you start getting the XCOM/Xenonauts levels. After a couple of steps there, you get an occasional RA2 level but most of the stuff is XCOM/Xenonauts level with some of them being gates that are required to be completed to unlock the RA2 level techs scattered throughout the tech tree.
Once a faction's gone up the scale, then after a small delay the other factions have an easier time getting to the next level in the scale, to represent the fact someone's cracked how to do the stuff so the intel guys sneak out a few samples, not enough to reverse engineer (without an actual action spent stealing the stuff at least) but rather to give clues on what the path that needs to be taken to advance are. And also so that the other factions don't get completely steamrolled because they don't have time to try and catch up at least part of the way. Wouldn't really be particularly fun if that's the case, and part of the reason we play these is for fun after all.
Basically the idea being that at first the scientists and engineers aren't really sure what they're looking at, so they are just finding the stuff similar but better to human tech. Then they get enough experience and can really start to play on the same level as the aliens. With the occasional flash of insight that means they pull off something that leaves even the aliens going 'Okay, I don't know how the monkeys managed to get that, but I want it so bad!'... of course they have their full tech trees so can counter that with something else they have but we don't (yet)...
In gameplay terms, it basically would slow down the initial stages of alien tech so the other factions can go 'Oh wait, someone's pushing the alientech hard, better get started on that, can't let them get ahead'... whilst still giving an advantage to the first group to crack the techs... Also allows game-changers to be discovered, but makes them something you have to work for, and thus something that once one faction obtains the others also have to work for a counter, instead of it just being 'Team A develops Superweapon Alpha in Season 6. They are dominating. Team B develops Superweapon Beta in Season 7, They are equals with Team A, Team C being crushed. Team A develops Superweapon Delta, Team B develops Superweapon Echo, Team C develops Super Weapons Foxtrot, Gamma, Hotel in Season 8. Team C is dominating.'
Kinda feels cheap if the crazy techs are everywhere and one is easily, quickly and cheaply countered by another. But I also want at least some crazy techs being thrown around because that would be fun/funny and not just 'Humans fighting on Earth using Sci-Fi weapons' which pure XCOM/Xenonauts level would be, and just pure low-key techs wouldn't have the feel of how alientech should be more advanced than human tech...