Stacking extra attack

[JellyPooga]

Alright. The rogue gets +2 AC from cover. That brings their 18 AC to a 20. Bully.

They still don't have much to do offensively, have very little presence on the field, have no resources or emergency buttons, and another class can do everything they do and more.

Everything you're saying can be true while at the same time be true that rogue is underpowered.

Underpowered or misrepresented?

3/4 cover offers a massive +5 to Dexterity saves against the DC:21 breath weapon of an adult red dragon or the less respectable DC:16 of a Beholders Slowing, Petrifying, Disintegration or Death eye rays, to call out two of the foes you claim such an action might be considered ineffective against. Even the +2 from half cover is a solid buff; Bless offers the same (average) bonus and is considered a solid buff, even into higher levels. Generating that as a bonus action, whilst also getting to do something else on your turn is no small thing, if only because no-one else is even going to think of doing it because they're too wrapped up in whatever resource dependent button they want to push next. Don't forget also that these aren't just personal buffs for the Rogue; the effects terrain can produce range from party wide buffs to permanent debuffs to plot-critical devices. Getting to produce that effect and similar ones, albeit situationally, without recourse to a limited resource such as a spell slot that could have been spent on something else, is entirely the point of the Rogue. They're a force multiplier for the resources of the rest of the party, whether they're saving a spell slot spent on Knock by picking the lock instead, by being a prime candidate for buffs like Haste or Invisibility (Rogues get more bang for the buck than almost any other Class from many, if not most buffs), or simply by being exactly where they need to be, precisely when you need them; no "I'm out of Ki points", no "can we just short rest first", no "I didn't prepare that spell today"...just all day, every day, reliable competence. If you see that as underpowered, then I'll say it again; we play very different games of D&D.