My view is that a player who follows the character build rules, and who doesn't make any obviously sub-optimal choices (in D&D, this would be something like ignoring all the hints that a fighter should start with the best armour s/he can afford), should produce a PC that is as meaningfully able to participate in the game as any other PC.
How that actually cashes out is going to depend pretty heavily on what is meant by "the game".
If "the game" means overcoming challenges - especially combat challenges, but also perhaps social and exploratory challenges - then that is your measure of balance.
I recognise that, in some (many?) RPGs, "screen time" can be as significant to balance as ingame success - and hence mechanically weaker PCs can still be balanced in the relevant sense. But D&D has never really supported this idea, because at least historically it has had pretty narrow and brutal success conditions (kill and loot, or be killed and looted).
How that actually cashes out is going to depend pretty heavily on what is meant by "the game".
If "the game" means overcoming challenges - especially combat challenges, but also perhaps social and exploratory challenges - then that is your measure of balance.
I recognise that, in some (many?) RPGs, "screen time" can be as significant to balance as ingame success - and hence mechanically weaker PCs can still be balanced in the relevant sense. But D&D has never really supported this idea, because at least historically it has had pretty narrow and brutal success conditions (kill and loot, or be killed and looted).