Biologicals have innate repair capabilities that require minimal resources to function barring major damage, and they replicate themselves efficiently. No reason to not go biological really.
Well there are actually lots of reasons not to. The inherent degradation over time, multiple fatal or crippling weak points, many parts that don't self-repair and can't be easily replaced, a mas production method that tends to produce lots of non-combatants, environmental tolerance limited by precise chemical interactions... The list goes on, but that doesn't mean they are a bad choice, just that there are other considerations. You are correct that they are usually low-maintenance and self-manufacturing which are nice boons, can we get better from elsewhere though?
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Slime-monsters? Dividing and recovering based upon mass is, hrmmm... Jellies tend to be bad at mobility. I guess we could try for some gas-slimes for an aerial force, though I can't see them being fast without magic... Aerial slime cavalry? Riders using wind magic to propel slimes that work like air-submarines that eat anything physical that they come into contact with and aven't been trained/treated to tolerate?
Elastic oozes can probably cover ground quickly if the terrain has something to grab hold of.
Hrmm, what else self-repairs and self manufactures... Ooooh, what about one of those monsters that make a new one from any pieces that are chopped off? Something like that would have to run more off of magic than biology, unless it was a very rare sort of worm, and even then, there are limits to biology.
Still, Worms and bacteria would be the go-to critters for the maintenance and reproduction side of things. Which, again, are bad at mobility, unless you apply wind magic on bacteria or have some sort of turbo-worm land-shark thing...
Or you could do it by focsing on the maintenance instead of avoiding it., Like, building Frankensteins/Necromancers and having the "units" be the maintenance and the "creatures" being a by-product...