I went Vortex to Neo. One is 3% accurate the other 1%.
I didn’t have a massive jump of FTP but having used both pretty consistently I’m quite confident my performance was overrated by the Vortex. Too many variables such as tyre tension etc. to make the FTP innacurate.
I not sure how the Neo would under report your FTP. There’s very little to affect the data it offers.
I was lucky as in between Vortex and Neo I had a back operation so couldn’t ride for a while, I used that as the reason why my FTP dropped. Makes me feel better about myself rather than admitting I wasn’t as good as the Vortex said I was.