The problem is the combination of note length and the actual note. Add in multiple notes played at once and you hit the 3 key limit of most consumer keyboards before most groups would be finished.
Beyond that there's also just the difficulty of mapping between the keyboard and keys, how do you easily deal with the gaps in the black keys, there's enough keys to map one to one with most of a piano but keeping that in memory while notating music is a big ask.
Sure, but that's not what the parent I was responding to was suggesting. gbc0 was specifically talking about using a 104 key keyboard to write out music, so I addressed that option and it's pitfalls. Other people have addressed issues with the midi keyboard option though.
I can't really address that very well because it's beyond my experience, but I'll summarize what I've seen. The main gripes seem to be it's still a little slow to enter notes because you have to specify the note length each time you want to change it and that it takes up a lot of space compared to something like this where you can just write it out when it may or may not be your instrument.
Beyond that there's also just the difficulty of mapping between the keyboard and keys, how do you easily deal with the gaps in the black keys, there's enough keys to map one to one with most of a piano but keeping that in memory while notating music is a big ask.