What you are missing about wyches is that they were designed for a very different game.
In 5th edition the idea was to get them into combat as soon as possible and then hopefully win by causing more casualties than they received. This worked because wyches were (and are) very difficult to kill in close combat, due to a) hitting first, b) a 4++ save against all attacks, and c) a further 4++ against most attacks from Feel No Pain (assuming they had a pain token). They weren't very lethal, but they could count on causing a few casualties due to a) lots of attacks, b) combat drugs, c) the 'No Retreat' rule, and d) a Hekatrix with an agoniser.
Arguably that is still what they do, but 6th edition chipped away at them in a lot of little ways that add up:
* Overwatch, which is lethal to them. Can be mitigated in some ways, but still.
* Agoniser is now only AP3 and
can be neutralised through challenges.
* FNP now 5++ instead of 4++, still won't help against S6 and higher.
* 'No Retreat' rule eliminated.
* Probable assault range reduced, making it harder for them to get stuck in quickly and more likely to take shooting (which is also lethal to them).
* No assaulting from reserve, including WWP.
* And so on.
Add that they still cost 10 points (12 with HWGs) compared to a kabalite warrior's 9, which made perfect sense in 5th ed. but not in 6th, and you see why wyches seem as though they ought to be better than they actually are. It is because they were designed to be.
Mind you, some people are still using them to great effect, and not only as suicidal vehicle blower-uppers. And fair play to them!
House of the Bitter Laugh, uniting the Kabal of Scorn, the Cult of Disdain, and the Coven of Jaded Atrocities