I've built my own mid-drive & hub-drive ebikes since 2008.
I used to think that converting old mt. bikes to ebikes would suit my purpose, but the old frame geometry & capacity may not be well-suited for extra weight of motor, controller & battery; finding locations to fit those items can also be difficult.
Nowadays, (as I get older) I prefer just get complete ebikes that are reliable, practical & suit my purposes, leaves me more time riding than tinkering with batteries, wirings & controllers.
Technology have trickled down enough that complete ebike packages under $2k have enough performance that are well capable & reliable that suit about 80-90% of people looking to "get in shape" and enjoy cycling.
No, they may not be suited for aggressive off-road at higher speeds; probably don't even have the optimum frame geometry to do high performance type on or off-road cycling.
But I suppose most people looking to "get back in shape" are not looking to do 2-3 hour rides average 25+ mph.