If you REALLY know the chord changes to a tune you should be able to arpeggiate over them in any key.
I set iReal Pro to ”numbered notation” and tried it in all 12 keys. I found I could memorize and play the melody, and even memorize the guide tones I wanted to emphasize (in green in chart below)… but I really struggled to arpeggiate in certain finger positions.
Here I am struggling to arpeggiate starting on the guide tones for On The Sunny Side of the Street in key of B and E; watch the fingering I chose carefully…. my intonation is simply awful in these positions.