Oh hands down. It's easy to build small increments of guard, but only 3 skills (2 of which are only on the legionnaire) can build a full guard bar instantly. 2 of those require an entire group of enemies to manage. Counter Attacks the only skill that can, like Barrier, instantly apply full guard. That's only on 1 of the 4 warriors and has a minute long CD. War Cry, Unbowed can with enough enemies nearby. Bull Charge can if you hit enough enemies in a line but... again it would still be weaker outside of a 100 or so war promotions (well, warrior 'and' rogue promotions as ranged would still tare you a new one instantly otherwise).
As far as guard becoming overpowered because of armor thats true in single player regardless of how much guard you can build. You could play a 200hp war with a whopping 50 guard max and if you only take 1 dmg from everything due to absurd armor values your still immortal. There will be enemies that can heavily cut into that. Archers all have abilities that ignore your armor - in fact every move they do that's NOT there basic shot heavily impacts your armor totals. They can go from doing only 1-2 dmg to you, to all of a sudden hitting you for 200 dmg because they did a little 3 run and skip move and quick shot.
Frankly I'd like to see guard become 100% HP and Armor values get rebalanced in single player all together since they're a tad absurd atm. but this being MP only board, and the only part we're talking about they could hopefully make guard be 100% HP in just MP alone. That would go a long way to making it not so funky. Doubt we'll get that but you never know, they did a lot of balance changes in ME3... some of those effected single player too though. Not sure if anyone remembers this but on release tech combos could only be done on a kill, not at any time. Half a year out and they changed it so they could happen at any time to match up with Biotics. All of a sudden a buncha completely useless classes became extremely viable, I kinda feel warriors are in that boat for the most part right now.