There are no songs that you cant pull off on an arranger ! What you are saying is that you are not comfortable with programming your keyboard to achieve whatever outcome you desire. Thats a statement of your skill level and has nothing to do with the limitations of an arranger or any keyboard or instrument generally. If you can program your instrument the arranger is literally limitless.
Well said, Spalding.
I haven't run into a tune I can't accomplish on an arranger. At one time arrangers wouldn't play On Bass (or bass inversions if you like) and multiple time signatures weren't possible.
Nowadays, with software by Michael Bedesem and Jørgen Sørensen (to name but two) and advances in arranger design, those issues are taken care of admirably.
As far as pros using them?
They are used in Nashville for songwriting; Martina McBride, Sting, David Paich(Toto), David Bryant(Bon Jovi), and several other prominent musicians use them in their studios.
Many OMB performers use arrangers, including several on this very forum.
I have done several projects with my arrangers, and these were done far quicker than with a workstation (well, the arranger is a workstation too) and the results were totally acceptable by my clients.
Arrangers have come a long way since the bingity boingity cheap sounding home organs, Casios and PSS instruments, and are only limited by a player's skill and imagination.
Ian