I misspoke in my original reply to Ulrik in this thread: tambourine is of course present in the General MIDI percussion map, but it’s not part of either of Dorico’s default drum sets, neither ‘Drum set (basic)’ nor ‘Drum set (full)’. You have my unreserved apologies for my mistake.
Sibelius has some logic to add unmapped notes to the drum set staff type when importing MIDI files, which allows it to handle notes that are not part of the staff type: it simply assigns them to the first unmapped notehead type on the bottom staff position, and then works its way up through the noteheads and staff positions. This ensures that every note is imported, but doesn’t produce any kind of sensible result, and you have to then spend a decent amount of time not only editing the staff type to make sure that the new instruments are mapped to sensible noteheads and staff positions, but also to editing the imported music to move those notes to the appropriate staff positions, and change their noteheads. I’m not sure this really represents a particularly efficient workflow, though I grant that in the specific case of the tambourine this is mapped to the middle staff line with a triangular notehead in Sibelius, so assuming that’s where you think it should be, there’s no post-editing work to fix that up.
Dorico, however, doesn’t have any logic like this in its MIDI importer. It will map notes that it finds onto matching instruments in the kit, but it won’t go and add new instruments to the kit in order to import any notes that do not have equivalent mappings in the kit. This is something we hope to return to in the future, but it is not something we are working on at the moment. I’m sorry that you find Dorico to be “very poor” and “quite bad” in this regard. I think this is putting too fine a point on it, since Dorico does, for example, correctly produce open and closed hi-hat markings when importing MIDI files automatically, something that Sibelius doesn’t do. I think it would be fairer to say that Dorico and Sibelius take different approaches to the importing of General MIDI percussion, and you prefer Sibelius’s approach.
I will have to go over our previous correspondence to find out what I apparently meant by asking somebody in Germany to come back to you on this, because I have no recollection of that. I apologise if I gave you the expectation that somebody would be in touch with you and you were subsequently disappointed.