I had a Vortex back in late 2015(?) and through a decent chunk of 2016. FTP and power, for me, was likely overstated by 30+ watts. I switched to an Elite Driveo + Quarq. After about 6 months on the new setup, I was convinced my old Vortex (I can only speak to the quality of mine) had skewed nearly a year of training data. My workouts on the Vortex were plenty hard, and I did get stronger – but the numbers produced had no relationship whatsoever to my Quarq-fitted bikes (2x), Driveo, 4iii, etc.
The Vortex got worse the hotter it got. Toward the end of a 90 minute session on the Vortex, the drift might be 40+ watts. So that’s really the issue: My Vortex wasn’t just ‘off’ by a certain number of watts – it drifted in the midst of a workout. On the final intervals of a hard workout, where the numbers really do matter, the hot Vortex was off so many watts that stuff which should have been mind-bendingly difficult, was actually pretty easy.
TL;DR ; if you can afford a new trainer, get one.