Poorly written and a mistake from a bigger narrative perspective, sure. Overrated? Not really.
The Void Elves could do some serious harm if they ever let loose. Their quests throughout BFA made it clear they were holding back, trying to minimize casualties, going for specific targets rather than slaughtering huge swathes of foes. Their goal was to break the morale of their foes, deprive them of vital information, and break the chain of command to keep them disorganized.
For the lore nightmare that they are, the Void Elves were still effective at everything they did. The Horde should be breathing down Lor’themar’s neck for having removed them to begin with. The sheer amount of damage the Void Elves have done to the Horde during the Fourth War was tremendous. I imagine the only saving grace, the only reason Lor’themar hasn’t been held to account for banishing such a devastating military asset, is that there are so few of them that the Horde in general has underestimated them.
Which suits the Void Elves just fine.