The demolisher is of course a very strong unit when used correctly.
Against Demon Princes, Wolfriders, Warbosses or other T5 targets it causes instant death.
It has a very good chance to pierce
LRs with 10+ best of
2d6
He can be a psycholical factor preventing some enemy units to come too close to him, you can use that to your advantage.
Often he will be one of the enemies main targets, you can use this to your advantage too though sometimes as hes still AV14.
His limited range is a serious matter though.
If you rely heavy on Plasma, Melter and Demolishers you will be very strong at 12-24" but other Armies like Tau,
IG or even some
SM lists might outgun you before you come close.
Because of this I find him a good addition to an army that lacks a hard punch at close range.
So still outgun your enemy on long range but if he comes close: send the Demolisher.
Don't expect him to act best against Terminators and such though as spacing with large models will make him hit only 1 at times. Against those Plasma and Lasgunfire or of the
LRs the Punisher are still the way to go.