Quick answer: only for simple, single-note lines (vocal melody, sax solo, etc.). For chords, drums, or full bands, the output is garbage – lots of ghost notes and wrong pitches.
Tested three “free YouTube to MIDI” websites. None delivered usable MIDI for rock, pop, or orchestral music.
Here’s a draft review for a “YouTube to MIDI free” tool or service. You can adjust the tone depending on where you’re posting (e.g., Trustpilot, Reddit, personal blog). Title: Works as advertised, but manage expectations