There are restrictions, but they're quite flexible. I have a main PC and a couple laptops I switch between, and have no issues.
You can activate 2 systems permanently, which is managed simply from the web UI. (revoke activations)
Other systems can be activated temporarily with your email/id + pass on startup.
FWIW, the small team has been fantastic and responsive since the beginning. (I've been on the betas and helped develop some of the early controller scripts. check the community pages to see what we've done)
I got a NFR license for helping out early on, and ran into the activation limit initially. (testing on all platforms, etc...)
I emailed them a few times to deactivate my current systems initially, then they added the option to their account management page.
If its a deal breaker, I suggest emailing them and seeing what they say. It wouldn't surprise me if they upped the limit or did something to help you out. They're an awesome group who really want to work with the community and users.
Not affiliated besides what I mentioned above, just a happy user. Best of luck, if you have any questions PM me or reply here if you think relevant to others :)