I have to disagree with you here. I frequently move Thunderbird from one new computer to the next and it's quick and easy. Just install the application, then copy the profile folder and subfolders over to the new computer. When you bring up Thunderbird again, all of your accounts, contacts, plugins, and email databases and folders are ready to go. It's also very easy to set it up for multiple computers accessing the same servers. I have my home, laptop and work computers all sharing the same emails without confusion or difficulties.

As for the robust nature of the databases.... In 10 years of using Thunderbird, I've had one database crash, and that was in an early version (3.x I think). They are up to version 12 these days and I have not seen a crash or problem in years. I have dozens of folders with thousands of messages archived in each.