Are you referring to a hardshell case?
Assuming yes, I have tried various of these in the past. Although still usable, they eventually all crack in the corners, but the best I have come across in terms of fit, finish and appearance is the Moshi.
Logic says that such cases should impact cooling, but my attempt as quantifying this by running the Cinebench test continuously with the Moshi on and off was not conclusive, so I think the effect is less than might be expected.
Some of the very small USB-C/USB-A adapters like
this one foul on the bottom case, and I had to trim a bit of the case away to enable it to seat properly.
In the 20 months I owned my MacBook I mostly used it without the Moshi, due to concerns about cooling and because it reduces the aesthetic appeal. When I sold it after 20 months of daily use the machine was pristine, except for shine on the keyboard keys. I travelled quite a lot with it in a shoulder bag, but I took care to avoid contact with other objects.