ZFS rollback snapshot general question

Recently upgraded my 14.4 installation to 15.1 and something went very wrong, so I decided to rollback the snapshot the installer script made for me (THANK YOU).

When I hit enter on the rollback command the screen went completely blank, there was some brief disk activity, and the keyboard was unresponsive (like no caps lock response etc). 

I waited a few minutes and hit the power button, then powered back on and... it worked! Back to 14.4 

My question is, I'm thinking that's not the expected behavior and I did something wrong? How and when should the ifs rollback command be invoked, and what should I expect to see?
I was logged in as me (regular user) and did su then zfs rollback.  Thanks!