The first Mac Minis allowed a CPU upgrade (if you were brave enough) as well as RAM, disk and optical drive. Obviously you could only put in what the motherboard supported, but it was a step in the right direction. Even the 2012 model is okay, I've got mine up to 16GB RAM and two hard drives totalling 1.5TB, the latter using the guides on this very site.
For me, the killer decision to not by an M1 Mac Mini, along with the non-upgradable disk and RAM (meaning I cannot buy the base model now and upgrade on the cheap in 6 months) is this : https://discussions.apple.com/thread/252041107?answerId=253977628022&page=1