I have hated the equal tempered major thirds for decades. I asked Wally Carroll to tune my Anglo in 1/5-comma meantone. (Partly due to discussions here and other places, so thanks!) It matches standard equal temperament at D, so A is ~2 cents flat and G is ~2 cents sharp. The major thirds are noticeably sweeter. I quite like it.
I don't notice that it clashes with other instruments, and no one else has commented on it either. I am generally playing Irish traditional, where the key center is near D and pipes and fiddle don't use equal temperament in the first place.
The wolf interval is G#/D# and is intentionally not even available on the same direction of the bellows on this one. Now, I do have another instrument with a slightly different 1/5-comma setup where the wolf interval is C#/G# and can be played, and I would call it "awful"!
I was tempted to try 1/4-comma meantone to get the just major thirds. But the discrepancy with others is larger. And, the fifths are very commonly used in Irish concertina, and are noticeably flat in 1/4-comma. So I'm not sure it would work out well.
Anyway, in my experience so far I would certainly agree that 1/5-comma is a good compromise and not extreme. Would recommend, would let it marry my sister, etc.