I've never had any issues with the phones locking up. The firmware we run on the handsets for our production system is an older version. I'm testing fusion right now running the 2.0.5 firmware and it's been stable thus far.
Been evaluating an X3 and X4 in the last year with Fusion. Never had any lockups, latest firmwares. Compared with other phones under evaluation from Grandstream and Htek, only issues with the Fanvils has been with screen glare compared to the others, no matter the adjustments. I'd recommend them!