I was never a fan but I guess they could be a pretty good choice if you wanted something to upgrade one piece at a time or something...as long as you started with the power supply. The ones I've seen have used the standard ATX form factor and power supplies at least.
The difficulty with making a sleeper build is the cooling. Those cases are not well designed for that. But you could build a low end system using one with minimum cooling requirements and it would still be 1,000 times more powerful than the original. I just wouldn't try to stick a couple of GTX 4080s in it or anything, lol.