As I just said in another thread, I'd prefer if all devours were 100% but had specific conditions that need to be fulfilled. The only way you can "counter" a random devour, no matter what percent, is to prevent it 100% of the time, so there really isn't any true counterplay with devour, other than "be immune or don't get hit", with the "be immune" options being pretty poor in general. True counterplay would have the mechanic able to be "played around" to either prevent the conditions that could lead to the devour or prevent the troop itself from casting rather than leaving it up to RNG.
Of course, then the AI would be garbage at using it and it would be severely gimped on defense. By being having "random" stuff that you have to stop the troop from casting 100% of the time to reliably and consistently defeat it, the devs have created some measure asymmetric gameplay (intentional or not) by artificially increasing the value of the AI's turns, because any "random" element always favors the AI because they don't have the burden of needing to be consistent and they start in a random and often severe disadvantage state most of the time. If you let a devourer cast, you lose a troop, which puts pressure on you to make the right moves. Randomly you don't die if you let it cast, and you also don't die if it targets something immune, and having full immunity generally means using terrible troops. Unfortunately, if you physically can't stop that coin flip, then the outcome is pure RNG, which makes the only valid strategy looping, and rarely leads to unwinnable starts (even worse for lower level players, who don't have a lot of options for troop choice, traits, and not great surge chance). The same thing extends to any random instant death type attack, and even to mana drain, which hedges its bets on stalling and board drop RNG (which is why it is also unreliable on offense if you don't also have a generator, but AI teams can still sometimes get away with it).
Without these mechanics and with the current power level of troops where they are at today, the AI would almost always be in a state of disadvantage. I think the actual designed intent here is to actually force you to deal with the consequences of the spell, eg., every x battles, you end up fighting a 3 vs 4-5 battle which puts the player in disadvantage, but it just doesn't ever work out like that. A lot of people are just unwilling to even attempt pull themselves out of this kind of hole (and sometimes, its not even possible). If you use a team where you allow this to happen as part of the norm, the fight drags on and on and with gameplay weekly reward quotas (PvP tiers, seals, snotstones) you end up wasting valuable time. This is what is generally meant when a team is said to be "not viable" - not that it is impossible to win, but that it won't be consistent because it doesn't have enough speed or control to deal with these mechanics.
I actually think the game as a whole would work better as a whole if most battles were designed to be asymmetric instead of trying to pretend to be symmetric battle between two relatively equal forces. Its almost never a symmetric battle between two relatively equal forces - AI attempts to gather mana to roll for your destruction, and you attempt to kill them before that happens either by using closed loops or lining up skulls for them while you charge up your board mod. AI improvement won't ever make it a symmetric battle, but it may make it easier for the AI to get their coin toss, which will only worsen complaints like this one. You can only use an "honest" team if they also use one, but neither side is obligated to do this.
If there were different, non random, varied and AI only hurdles to overcome I think it would make for better gameplay in the long run. Things where they can still be a constant, guaranteed threat, where they would be considered "objectively overpowered" otherwise, to apply pressure for you to make smart moves, possibly changing what a smart move is. Where you can pass the turn and have some give and take in some situations. Things that only appear sporadically (having every fight be an uphill battle isn't fun for a lot of people either) and differ in the way you tackle them, rather than being a player choice leading into a "defense meta" where you are tackling the same "challenge" over and over and over again at least every third battle in any given months long stretch. Usually by looping. Sometimes by using a specific troop. Sometimes both. (On a personal note, I'm sick of Deathknight because I never particularly liked using Khorvash all that much and I hate stun being a panacea for dealing with all trait based stuff and hope future "challenges" have ways to play around instead of requiring a full nullification brute force to avoid that instakill diceroll.)
Can't see this happening for normal PvP though, unfortunately. Maybe as its own game mode, though...
tl;dr - consistent, condition based 100% devour + not random but "overpowered" AI only based obstacles to overcome that have some room for give and take/counterplay outside of straight up losing troops