IMO that wasn't the case at all.
The interface itself was fine and relatively easy to use.
My issues were underlying.
I had a 500 that largely worked just fine. I would sometimes randomly just factory reset on me. I broke it in a crash.
I got it replaced for cheap by Garmin, but I ordered a 510 since I didn't want to wait for the RMA process.
The 510 had _regular_ factory resets out of nowhere. Like every handful of rides I would power it on and all of my settings/sensors were gone.
I learned to live with it until it just started refusing to sync GPS. So I RMA'd.
RMA 510 worked fine, then suddenly factory reset issue started after a firmware update. Then it started having GPS issues.
Got Garmin to RMA the second 510 and replace it with a 520. I sold it immediately.
I've since moved on to Stages Dash. I have two (one dedicated to my indoor bike) and they have been flawless. Not a single issue since I bought them.
My only complaint is that they never released the mapping features they promised. But I give them a pass on that as they realized it simply didn't have the horsepower and didn't want to shove a crappy feature it that would suck. (Hint: Garmin, fix the core features before releasing dozens more...)
I have a Stages Dash M50 arriving today. It has all of the promised features of the original Dash and more. Time will tell if it stands up to the quality of the original.