^ All good points from Ndbowhunter.
Also, when calculating your overall length, be sure to always subtract the distance of the barrel threads.
Because even if a barrel is 11.5" inches long, you can't go by that. Because those threads go back about a half an inch further behind the muzzle that you're inevitably losing once the can is threaded into place.
And also, you would need only need to pin weld the mount, but also P/W the tube onto the mount unless the can was a welded design (I'm not sure what yhm is doing these days---they used to only do monocore stuff).
There aren't really any downsides, but there are some things you need to be aware of. Replacing barrel nuts or handguards, gas blocks and such would not easily be interchangeable anymore. So, make sure the upper is set up the way you want now because changing it at some later date in the future will be a pain in the whazoo.
I'm not saying it's impossible to change anything because it's not, but you'd have to mill out the old pin and weld and redo it, etc.
After the atf got stupid about pistol braces, I decided to permanently attach a can onto my scorpion and as a F' you to the atf, I then added an actual buttstock to it out of protest that I wasn't going to be paying them any more tax money for another stamp.