I am a former professional machinist/fabricator, and am quite satisfied with my mini mill and mini lathe for my needs. No, I wouldn't want to use them solely to make my living, but used for small subset of my living they work much better than the nothing I would have otherwise.
I'm limited on space and electrical availability, and until my fiance and I just bought our first house that we will be moving into on the first, we were moving from rental to rental, and full scale machines were absolutely not an option for me.
I found that the g0463 mill and 9x20 lathe were the smallest that would really work for me while still being able to be easily moved with an engine hoist and a pickup truck.
Although I would be hesitant to move to the smallest mini machines, many folks get plenty of good usable work done on them.
They do require experience, tinkering, finesse (for pros and amatuers ) and/or patience and learning (for those completely new to machining and both pros and amatuers) to get the best out of them.
Many things were mentioned above, but the sky's the limit... I even used my mini mill and lathe to build a professional quality 2x72 grinder. You can make knife parts but you can also fabricate and repair knifemaking equipment.