I feel like there are multiple instruments missing from Musescore.
Most notably, a gunshot instrument in percussion. This would be necessary for the 1812 Overture to be made on Musescore as it uses 16 cannon shots, and a gunshot would fit that requirement.
Another is that there are more than just 1 kind of bagpipes. There are actually more than 10 different kinds of them, and only 1 kind of bagpipes, just called Bagpipes in the software, is available for use.
Another thing as well, I've seen many scores that require instruments, mostly brass instruments, that aren't available on Musescore.