You mean like those bootstrap gate drivers? Those need a supply on the "flying" side, and it can only "fly" "above" logic ground. Never more than, say, -6V below, and they're not functional while below.
This is fine for the intended switching application, because the switching node is always coming back down to ground periodically, where the flying supply can be recharged with a diode: a bootstrap supply. Also, with logic ground = power ground, the flying side never goes below ground more than a little bit (due to transient overshoot).
An isolated gate driver IC is just another opto, or logic isolator or whatever, but with extra guts and expense. (And yeah, if the output drive capacity suits your loads, that can be very handy.)
As for digital isolators, those are usually 3.3V, what's wrong with that..? Okay so the output side isn't, but logic interface is very easy...
Solenoids: are they not better to drive with an open-collector (or drain) switch? You can get arrays to do that (like the ancient ULN2003 and such), and you can save on isolator channels by using a deserializer (usually a 74HC595 shift register, or any number of SPI IO expanders).
Tim