I dont think iMovie is just for beginners. Its a great start, dont get me wrong. But there are still a number of very large YouTubers that use only iMovie, because its basic and does the job! If you only want to throw clips together, with some nice transition effects, the odd PIP, and some titles etc, its great. Nothing else is needed! Vloggers, for example, have their raw footage which they just want to edit together andget uploaded, and for those its great. You can even do green screening etc on iMovie - its more advanced then people make it out to be TBH.
Although I use FCPX, I did use iMovie for years, and go along with it perfectly fine. The only reason I moved to FCPX was because I didnt like working with the AIC codec, since it produces graining and banding on shadows or darker areas, whereas ProRes is a more professional level format, and produced higher quality exports.
If using iMove for well lit video, or video recorded in daylight etc, iMovie's quality is great.