Whenever massive arguments like this break out, my method for determining what works best is to just completely strip away all the fluff of the scenario and look purely at the mechanics in play. I see where the arguments are, I determine how I feel about what the mechanics allow for, and then I make a rule. After that, I put the fluff and narrative back into place and handwave any minor discrepancies in the narrative because the entire game has discrepancies between narrative and mechanics all over the place and if I'm willing to handwave it over here, I can handwave it over there.
So the questions become:
- Can a Rogue get the opportunity for extra Sneak Attack damage almost every round and is the game balanced around this idea?
The answer for this is 'Yes'. By being next to an ally adjacent to a foe, you get to add SA bonus damage once a round when you hit. The game expects this to happen and thus it doesn't matter what the narrative of the scene is to generate this mechanical expression. You could say "the rogue gets to add Sneak Attack damage on any attack they make while singing
Rule Britannia!" and as far as the game mechanics are concerned, it is balanced for that.
- Can a Rogue attack with Advantage every round?
Right now the answer is "Quite Often", but when it occurs there is a cost associated with it. Rogues can get it via an ally's spell or class feature triggering it (Tripping Attack, Distracting Strike, Wolf Totem Spirit. Guiding Bolt etc.)... by using variant rules (Flanking)... or by spending a Bonus action and not moving for the round (Steady Aim).
So does a Rogue that uses a Bonus action * and * has to succeed on an ability check (the Rogue's rules for Hiding) unbalance the game? My own personal opinion is 'No'. With so many other ways for all characters to gain Advantage on their attacks during combat, I believe the game expects most characters to be able to get it quite often and thus the game has been built for this. In fact (as people have mentioned) the game was written specifically so that a Rogue
could use a Bonus action / ability check combo via Cunning Action to possibly gain Advantage each time they use it.
Thus... if the game mechanics have been designed for this expectation... disallowing it purely because (general) you, DM, don't like the
fluff representation of it... means that as far as I'm concerned, you are in the wrong here. There's no problem not liking the fluff, but it then falls on you to
adjust the fluff so that the mechanics can still be run as the game has been built for them. The Rogue has been designed specifically to possibly gain Advantage on one attack per turn via a Bonus action * and * a successful ability check versus a target's Passive Perception... so do whatever (general) you need to do in your imagining of the scene in front of us to justify using that mechanic.
If (general) you can't imagine any scenario of a Rogue hiding during a combat... just remember you ARE able to imagine that same Rogue at 7th level being able to
completely avoid being hurt while engulfed within a massive Fireball, Ice Storm, an ancient dragon's breath attack etc. via their use of Evasion and a successful saving throw. If you are willingly able to accept THAT
ridiculous fluff narrative to a game mechanic... you need to try a little harder on the "how does a Rogue hide during combat?" imagined scenario.
