You're correct that there have been reports of Thunderbolt-FW adapter issues, although I can't give you any feedback from my own experience. If I were you, I would try to find a way to test the worklow and connection prior to purchasing the camera. Any HDV cam should suffice.
Fortunately, you have a good fallback solution in first being able to capture to legacy FCP – then importing to FCPX. (BTW, did you mean to type Final Cut Studio 3?)
I have a 15 in. Macbook Pro with Retina Display equipped with Thunderbolt ports and using the Apple Thunderbolt to FW 800 adapter and a FW800 to FW400 cable to my Sony FX1 I have been able to capture both HDV and DV footage without any problems whatsoever. I assume this same setup would work fine on a new iMac equipped with Thunderbolt ports.
I have used an 800 to Tbolt adapter to capture HDV on my MBPro late 2011 17".
This should work just like connecting to a firewire port.
I've heard of some iMac users having issues but it's not confirmed that all newer iMacs have issues.
Probably a one of system drama or dodgey cable.
As mentioned already, nothing beats a test drive at your local Apple Store.