By all-in-one, do you mean one of those PC's where all the hardware is behind he monitor?
If so don't they basically use laptop-spec hardware?
Yup, they do. Expensive to buy, expensive to upgrade and definitely not gaming machines unless you are going to spend 50 to 100% more than a high end gaming rig, just to get mid range performance.
Then they get hot and throttle down to cool off. If one component breaks in them the unit itself is broken. Lack of upgrade options etc.
Yup, I don't like them.