Biased against a balanced, fair, fun sort of game, where no class choice is a trap. Biased, more specifically, in D&D towards casters and against martials. Thus the sad reality of the martial/caster gap in 5e, and LFQW, 5MWD, etc in prior editions going back to the beginning.
Indeed, that's the only reason realism, verisimilitude, dissociated mechanics, or immersion are ever brought up, to deny a player character that isn't defined as having supernatural powers an equal place in the game.
When someone points out that other aspects of the game, like hp, are also a problem under such restrictive ideas, it's like "yeah, but that's OK" - sure it is, casters benefit from the lack of realism of hp, too.
Were we to honestly consider how readily magic - lacking any yardstick for realism - could be balanced to, or below, 'realistic'/verisimilitudinous/associated/whatever takes on non-casters, we'd find that it should be a non-issue. Classes could be balanced in spite of demanding verisimilitude, but they aren't. If class imbalances were just, y'know, techncial errors, then you'd expect they'd be all over the map, not very consistently generating a 'gap' between martials and casters.
If the real issue is verisimilitude, then creating the reality-conforming classes, and arbitrarily limiting the casters down to their level should be fine. It never has been. Not once in any discussion I've ever seen. At most, we might get a nostalgic suggestion of restoring old-school limits on casting - forgetting how often they were ignored back in the day, anyway, because they just were never fun for anyone.