User interfaces in GPSs are poorly designed.
Like a really horrid VCR, they are OK once you have them sussed.
The basics e.g. simple route entry, tend to be fairly easy to work out, but anything beyond that (e.g. waypoint handling on the missed approach) vary a lot, with some potentially fatal gotchas. But then anybody using an IFR GPS must fully understand it if they use it for the said features.