Encrypt external TM backup drive?

I just restored from a Time Machine external drive to a new Mac. The drive had been used for backup on an old Mac that has died. The new Mac is encrypted; not sure, but I don't think the old one was. In setting up the new Mac I chose to encrypt it. What are the cons to encryption of the new Mac? Should I encrypt the external backup drive as well and, if I do what would be the downside? Would that prevent me from using it to restore data to a new Mac should I need to do that?