Unity seems to have everything that Blender has when it comes to animation, except for armatures and rigging. So is Blender useless for simple animations without rigging? Should I instead learn Unity’s animation system and forget about Blender’s animation system? What should I do? What is the best practice?
Learn both. Blender (not hands on experience) has superior animation tools to Unity. Simple non-rigged animations can be created in Unity. Also moderate complex 2D rigged characters can be animated in Unity.
However - I’d suggest using a dedicated animation package for all rigged stuff - especially characters.