I'm leaning towards the "it's too difficult to use" opinion. I've had a Nova System for about a year and am going to sell it and try a Line6 M9.
The Nova System isn't difficult to use but it's "too difficult" to use for what I want it to do. For me it comes down to one issue. It's that you have to navigate through multiple screens to access all controls for several effects. That sounds petty, but it's annoying especially when there are 3 or more screens to go through. You turn the knob to move through the screens, but periodically it advances too far and skips ahead 2 screens. You find yourself barely tweaking that knob to only advance 1 screen at a time. And besides , it's just annoying that you can't see all controls on a screen with a dedicated knob all at the same time.
I like how the M9 and M13 show all controls on the screen at once, with an active knob for each control. We'll see how the effects stack up. But honestly, I haven't found the effects to be that impressive on the Nova System. The distortions aren't great. I don't use them at all - then again, I won't use the Line6 distortions either. It will be in the loop. The mods didn't blow me away either. And the non-boutique delay pedals I have sound better and are easier to dial in than the delays on the Nova.
It's not exactly a difficult unit to use, but for me it's annoying enough to tweak that I'll try something else.
BTW, in the past I had a Boss ME series pedal. I still think that has the best layout of anything! Multieffects with a knob accessible for each selected effect at all times. Great! But the sound quality was awful. Avoid it.