Yes diode based isolators tend to display more voltage loss than relays, or of course wiring direct. This was especially true many years ago, but isolators have gotten better in recent years... not a real big issue anymore from what Ive seen. But yes, that was always my reason for chosing a relay over an isolator, Ive pretty much gotten use to setting them up with relays now. Relays tend to be cheaper than isolators too.