Too general a statement to take it as a rule. Recessed arm board may obstruct cueing device motion or impede VTA adjustment, etc, etc, but not in every case.
Recessed Tonearm Armboards
A tonearm manufacturer once told me that a tonearm is restricted in movement if the armboard is recessed below the plinth. Now I have never owned a turntable that had a recessed armboard, but I have seen some that were recessed a few mm's to an inch, but nothing more than that. I wanted to know what is the reason that would cause the restriction or its ability to move less freely. Or maybe folks feel that it's just not the case.