We have about half a dozen still on FM10 (who were all upgrades from 9) and haven't had any problems.
FMI should be able to give you a new license for 9 that doesn't require activation. They just did it for me a couple weeks ago.
I had a couple problems with 10 back when it was the shipping version. Once, a reinstall fixed it. The second time, I had to get a new license key ( full version vs the upgrade ).