Well I can't vouch that you won't discover a bug (grin)! I'm still a relative newbie to arranger keyboards and am strictly a home studio hobbyist. It seems like most of the criticisms about bugs that I've read come from users that really use the PA5X on gigs.
But by the way, I got the chance to sit down and play the Montage M8X at a local music retailer earlier today. I was curious to try out its control panel after reading some posts here claiming that it was significantly easier to navigate than that on the PA5X. Wow, the thought that went through my head was that either I or the poster must have been playing a Montage from a parallel universe. Because I found the Montage's panel to be vastly less intuitive. The person who last played it at the music store must have been trying to set up a multi-part performance (the term Yamaha now uses for presets). The mere act of backing out of that mode and calling up a simple preset was not remotely intuitive. I felt like I needed to have the manual in front of me to do almost anything. And this is coming from someone who used to own a Yamaha Motif. On the flip side, the quality of the Montage's presets were outstanding, and the keybed on the M8X was sublime, the best weighted action I personally have ever experienced on a digital keyboard. I came away seriously considering acquiring one in the future to complement the PA5X, despite the complex and unintuitive controls.