Teach me when to use Onslaught.

[AziraSyuren]

I'd disagree. So many other melee gap closers do not consume any resources. Look at DRK's Plunge. Zero resource and its 200 potency, where at Onslaught costs BG and is a small 100 potency. Far from being the best desgined gap closer.

That said, I am not denying that it is useful in many scenarios.

You literally just said the exact reason it's so well-designed as the reason it isn't. Most of the other gap closers are powerful and free enough to need to be used on cooldown for DPS. Onslaught isn't. It can actually be used as an effective gap closer when it's needed. The others can't, outside of planning the whole fight around them.

It's not really a matter of opinion.