Knee pain. You loose the advantage of your body and leg weight pushing down with gravity when on a bent...
Spinning become more Important.
Mashing become more dangerous....Partly because your using your engines differently.
The 'Bent Rider Online ' Forum people know a lot about these Issues..Go talk to them...
Factors I had to consider while doing my Tour Easy fine tune....
Seat angle,
Pedal to seat back distance,
Crank arm length,
Handle bar height and pull back.
Etc......

The ride for me Is completely pain free, I get bored or tired before I get pain but I don't ride that far.
Setup and fit is still critical but you must not forget that you are using your body differently so you will need miles to get your, 'Bent Legs'...

TANSTAAFL: "There ain't no such thing as a free lunch",
As Robert A. Heinlein said In one of my favorite stories... The Moon Is a Harsh Mistress