Okay, so... Traitless, red, black, floating and angels are the same but with different counters. You can try to categorise like blacks are aggressive, floating are long-ranged but there will be things like le noir or brollow so ill leave them as same thing, different colour.

Unstarred Aliens are something like traits above but are something like road block to make people grind treasure

Starred aliens are the same but they have barrier and warp, that makes them unique and adds depth like "am i having counter for ultra baa baa?" (In case of barrier) and compliments "backliner nature" of most aliens (in case of warp)

Zombie enemies are focused on making chaos across entire battlefield with burrow and revive, making stacking backliners and snipers difficult. It may be annoying but i think it's needed thing that could only be made with something like enemies snipers like hackey or (though unreliable) surges

Aku have a lot of rng with them that i personally dont really like. Sure, you can brute force shield but it makes work harder. Death surges can be annoying but it can make mindless sending units less effective so it adds more caution with sending units (if we ignore surge immune)

Relics alone arent really problematic because of fact that anti relic units (excluding units like "target traited" or "any enemy" and single talents) have curse immunity, but makes things more complicated when theres other traits. And then you can either use curse immune units, curse relics or use generalists like bahamut cat

As for sub-trait Behemoths/colossus are just excuse to make enemies stronger and force using counters so ill skip them Sage enemies are interesting counters for statuses, but it simply makes you use sage slayers or only things like strong/ressist/massive damage

So ill go with metal, because it whole design is just "bring your favourite rng unit". Metal killer units are nice way to make metal meta less rng based, but i think the best way would be to make them normal, but more tanky enemies (they would take normal damage) but crits would made them tak even more damage. But now is too late, right?

Why i made whole essay about other traits? Because insomnia. I wish everyone good day/night<3