Strange that balance is a controversial topic. Classes, powers and weapons need balance so that there are compelling alternatives rather than one or two setups that vastly superior to anything else.
Additionally, the player character needs to be balanced enough relative to enemies that the game is not trivialized on the higher difficulty settings. This is a problem with mid-late game leveled Shepard particularly in ME1 and 3, and to an extent in ME2 (although in that game special upgrades are at least as important as level and gear). Having multiple difficulty settings is a good idea. Allowing the player to become so powerful you can't hardly tell the difference between them is not.
I don't think Shepard should count. They were at least 30% cyborg by ME2, with the latest cutting edge technology some tech savant came up with after you acquired the schemata from secret labs. If you look at the scans and upgrades, Shep is basically Wolverine Light, with tons of biotic and tech powers. Even the biotic implant is at least a generation better than what everyone else uses (L5x/n, if I'm not mistaken), while Jack is using a special version of the L3, which Shepard used in ME1.
It's totally believable that Shep can knock down the Shadow Broker with bare hands, what's ridiculous is that loudmouth James Vega can hold his own. Tiny FemShep could kill him if she threw a fullpower punch. (Just like Summer Glau as Terminator.)
So yeah, Shepard is definitely not a standard biotic.
If we look at what NPCs showed us during gameplay, then biotics are fairly powerful and there isn't all that much strain, it's more a concentration issue. Samara/Jack kept up the biotic bubble in ME2 for quite a while, during combat and suffering a constant onslaught of swarmers. Once they were done with that, they were fine after a short moment to catch their breath.
On Thessia, a single Asari soldier kept a biotic barrier up for minutes while Reapers hammered on it. She didn't collapse and needed to eat after she was done.
Our darling psycho Liara killed her assistant in her office and only had this to say about her former employee: "She needs to work on her barriers." Fight to the death, but not breaking a sweat. And Liara is basically young and inexperienced compared to the standard Asari*.
I don't know, have we ever seen an exhausted biotic in the games?
Of course, ME science is space magic. It's fantasy and not internally consistent. Whatever the writers say goes, even if it contradicts everything that came before.
From a balance point of view... I don't mind being a special overpowered snowflake in single player. A full biotic team (Liara, Javik, Shepard) in ME3 was unstoppable, everything died before I could use my gun. I don't think people seriously complained about that then, so why do it now?
Multiplayer is a completely different issue and there it's okay to play as nonames. (Although we do know who the N7s, the GI, the Cabal and the Vorcha are).
Just an everyday grunt who only has 3 powers and usually can't afford to carry more than 2 weapons.
*Btw... Asari biology is BS. All Asari are biotic because the Protheans changed their genome and then dropped tons of Eezo on Thessia, so all Asari are exposed to it. That's all fine and okay... until they went to the stars and settled dozens of planets and lived on space stations and ships. They are still all biotics, with no exception. Even without Eezo exposure. Asari. Space elves, more likely.