The long version.
What Synthesia does well
Synthesia turned the piano into a game long before anyone else did. Notes fall toward the keys, you play them as they land, and the app tells you how you did. That is still the fastest way to get a beginner from zero to a first recognisable song, and it runs on almost anything: Windows, Mac, iPad, Android.
It also keeps a few things LumaKeys does not have: finger numbers on the notes, progress tracking per song, and a sheet-music view for the day you want to learn to read. At $39 once, it is the cheapest serious way in.
Where it stops
Synthesia is a player. It shows you a MIDI file and scores your playing against it. Once you can play the piece, the app has nothing more to say: it does not record your version in a way you can edit, it does not turn a recording of a real piano into notes, and its video export is a basic Windows-only feature that was never meant for social.
That is the point where most self-taught pianists move on to YouTube, a DAW, a video editor, and a folder full of MIDI files. LumaKeys exists to be the app they move on to instead.
What LumaKeys is built for
LumaKeys keeps the falling notes and adds everything around them. A piece comes in as MIDI, as audio that gets transcribed on your Mac, or as your own recording. You learn it with loop, slow-down, and hands split. You record your version, fix notes in the piano roll, play it back through your own piano plugins, and export a video in your colours. The repertoire keeps all of it in one place, searchable, by composer.
It is Mac only, and Pro costs more than Synthesia. Free covers the full app with caps, so the way to find out whether it fits is to download it and bring in a piece.