I found the biggest challenge with a pistol-mounted WML is exactly what you're alluding to: controlling the light independent of firing the pistol.
Using the trigger finger to activate the light means either (1) the light has to be switched constant-on before firing or (2) the light goes off in momentary mode as the finger transitions to the trigger. I concur that holding the switch in momentary mode while firing presents some challenges, but switching to constant-on could present some liabilities.
On the other hand (no pun intended), using the support hand thumb to operate the switch requires (1) both hands on the pistol and (2) a somewhat different grip than if I was simply firing with two hands. At least for me, this requires incorporating two slightly different grips depending on whether I'm using the WML or not. FWIW, my pistol WMLs have included the SF300 series and the Inforce APL.
Over the last year or so I've spent so much time shooting strong-hand-only it's almost become my default method. As an old Bullseye shooter, it feels natural and leaves my support hand free to operate a handheld completely independent of firing or where the pistol is pointed. I've also become comfortable transitioning among the different hold techniques for the light, so at this point the handheld works quite nicely for me.
All that said, being able to control both the pistol and light with a single hand could offer some advantage in specific scenarios.