I believe most characters do have a place in the game. But they don’t necessarily all have a place in every single Escape Hive. Fact is some characters fulfil quite different roles and these skills won’t necessarily be useful in all Escape Hives.
Like I said, Escape Hives have a theme and this will determine gameplay.
Let’s take The Malfunction as an example. It features lots of DeeBee Shepherds and there is a mutator which enables them to fire freezing bullets. Because of this, melee characters will typically have a hard time because using melee combat becomes very high-risk. That’s why a sniper like Fahz, Baird or Paduk would be a better option.
However on many other maps there is usually a role for a melee character, and Lahni and Cole are both considered very powerful in this role.
Or consider Melee Brawl - there aren’t really any notable explosive weapons on this Hive, so therefore Keegan is a fairly redundant character whereas on other Hives he is very powerful. This Hive also doesn’t feature many ammo boxes. This Hive has a specific design and gameplay loop in mind - melee combat.
The Hives you mentioned - The Onslaught; and The Gatekeepers are Hives which require high-damage characters. If all characters were suddenly strong enough to deal out such damage, then it defeats the purpose of having different characters and roles because they’d all be able to do the same (or similar) levels of damage.
Also, Horde and Escape are very different modes, and some characters are simply more suited to one over the other.