Vehicles that need to be seen like police, fire, paramedic, road safety, all have flashing lights. Indeed cats have flashing lights to use when they're causing a hazard.
I suspect you're right WRT difficult to estimate distance/approach speed for vehicles with only a flashing light, but not sure about combinations of constant and intermittent lights.
Bonus: in UK cyclists may use flashing lights but only on clothing IIRC, at least they need separate constant lights on their bicycles.