Accidentals.

Take an ordinary recorder. Play a tune on it in C. The twiddly decorations, the things that a folk musician who is not rigidly following dots do come from wiggling the fingers in a certain way. Ideally you want your twiddles to come from putting down or lifting up one finger at a time only.   Like the little bit everyone does on a D chord on the guitar, the top F sharp goes up to G, back down to F#, then to E than back to F#. Now do the same thing in A - still easy. Now E - not so easy because you have to get the little finger over to get the F# on the D string.

Now play your tune on the C recorder in (let's be brutal) F# or B major. All those little easy twiddles have become hard.

Same for a fiddle player - all the bits where they could double-stop with ease vanish when the key changes.

That's something of a red herring, Richard, in the context of the present thread which is about playing from written music.

However, there are some points that are worth addressing:

First; just because you are playing from dots doesn't mean you don't add twiddly bits. I certainly do whether I am playing from dots or not, but usually once I have the tune properly learnt.

Second; different instruments need different approaches to decoration so what you would do on a recorder is different from what you do on a fiddle. I have a wooden flute whose fingering is the same as on a whistle. Some decorations, or decorations in some places, I can do on both, but there are some things I can do on the flute I can't do on the recorder and some things I can do on the recorder I can't do on the flute, so it's horses for courses.

If playing a tune in a different key affects the way you decorate it, then so be it. You have to adapt. So what? It means you need to be thoroughly familiar with your instrument.