it depends on the device, some of them are old school magnetics and most these days are cheap switchers. Either one can have trouble with the "modified sine wave" inverters depending on their design particulars. The switchers are probably more tolerant in general, since they are going to rectify to DC as the first step. If it has a big transformer in it (you can usually tell by the heft) then the steep slopes of the square wave coming out of the inverter will cause it problems. You can now buy small true sine wave inverters for a pretty reasonable cost, no reason not to go that way.