don't think it's necessarily true that they can't be ganked, I just think it's about methodology, you need a different approach.
A champion like Poppy could be a good candidate for lane ganking instead of the standard route through river. No time to charge the ult up as you're on her before she sees you coming. As a fellow Kha'zix player I can tell you the lane gank is becoming a hugely preferred method for me, unless they're skull fucking your turret there's not a huge level of success on the standard river gank.
Ganks don't always have to result in a kill either, blowing flash, or at the very least forcing them out of lane provides an edge for your laner, or even just a bit of relief from the pressure of a strong lane opponent. Having been on both sides from all lanes I understand that even the jungler showing up to scare the opponents out of playing so aggressive can provide major relief.