That is true, but if you fail because nothing is getting done, no bonus objectives are being met AND the defense target is getting swamped by enemies, then kills are also an indicator. You can be support-oriented and CC enemies, and/or run around and collect glyphs instead of killing stuff. But if everything is collapsing something is wrong.
It took me 8 tries in total to clear the mirror defense this week, and in the final successful try most players just used their mechs anyway. But getting there really cost me some sanity.
That being said I am not fully against the randomizer, since it makes you play things you usually don't and you might even rediscover a weapon or frame you'll like. For example i made an Arca Triton slam build with melee influence that was pretty strong and fun to play - and without the randomizer I never would|ve touched that weapon again.