As reliable as anything else. I mean, honestly, everything sucks in one way or another.
Sure, cables won't suffer any electronic glitches, but chips don't fray or corrode. Is one better than the other? Well, electronic controls bring along a lot of other features that help make piloting the boat easier. This, of course, runs afoul the grumpy old man syndrome of "we knew how to do it the hard way".
Yeah, whatever. I'll pick throttles with selectable power curves over crankly old cables any day. Nice to have fine control during some situations, but sync'd rpms or faster controls in others. And having trailed a friend's Palm Beach 50 a few weeks ago, I'd trade out for the nearly no wake, flat profile he was cutting while clipping along at +25kts with pod drives. Using half as much fuel as my straight shafts.
For some having electronic controls is absolutely worth it. For others being able to service a cranky old set of cables and pulleys in the middle of nowhere, for nothing, has it's merits. Different strokes...