If you are a citizen of the United States of America, and you are returning to the United States from a foreign country, Customs and Border Patrol are not allowed to deny you entry to the United States of America.

Of course the caveats - if you, as the citizen, have an outstanding warrant, they can arrest you. They can take your electronics. They can delay you. They can put you in a room by yourself for some period of time. But they can’t deny you entry without some kind of legal reason. They can’t hold you indefinitely.

It just comes down to how much the citizen is willing to be inconvenienced.

And I believe any American with any felony on their record will have trouble visiting Canada. I once traveled with a guy who had a felony assault conviction from a college bar fight 15 years prior. They wouldn’t let him in.