Yeah, on paper is sounds might potent, that's a boatload of attacks that's going to overwhelm most things they aren't wounding on 6s. However, that's ignoring how difficult delivery of that unit is. Even with rerolls, making a 9" charge from deep strike is not something that can be relied on. If you fail that charge, you're massive wych bomb is a sitting duck and will get torn apart easily by shooting. Sure it's not a super expensive unit, wyches are relatively well priced but it is a lot of pts to chance on maybe making that charge.
I actually think if you wanted to make wych bombs work, I'd consider footslogging, something crazy like 80 wyches on the field. You'd really want to run cursed blade though, ignoring moral is big on those 20 strong squads. Would that list actually work? In many matchups probably not, but I've always thought wych spam like that would be an interesting possibility, though I don't have nearly enough wyches to try that.
The Night is Dark and Full of Terrors...