I've never played on a board where you can get "impossible to see" LOS between a Wave Serpent and it's rear arc. My gaming area doesn't play with thick ruins or intact buildings much and that's in general the only thing that I would imagine blocking los totally. I don't see them as a liability or a game winner. They are a very strong unit in a codex of very strong units. They are very flexible, which is what I like about them.
That's what the table edge is for. Park your Serpent's ass roughly 3" from the edge and it's impossibly to DS behind. Also, a lack of complete LoS blocking terrain is a HUGE issue that your gaming area could do with addressing, because it massively
benefits shooting armies, and they're already more than strong enough as it is, they don't need any further help. Trust me, you will
notice the difference if you add two to three large pieces of solid cover to your tabletops.
Stealth and Shrouding isn't enough to survive those hits? Really? A 2+ cover save will lose very few models on a volley like that, and if you are using Guardians or Avengers to achieve those numbers they will probably fall to whats left of the squad if you can't handle it. (assuming 10 DA's you will wound them 10 times on average and they will lose 2 models)
Firstly, if they just assaulted you, exactly what cover are they using? Secondly, that's only DE harlies (if they haven't been FAQ'd yet). Current gen Eldar Harlies roll to see using 2D6x2, which you can safely ignore with Catapults, resulting in 10 wounds or 6.6 casualties. Guardians and Avengers are identical offensively apart from a 6" range gap, and the FC initiative change means you're striking simultaneously in combat, so even 5 Avengers will get 10 more attacks against the Harlies, dropping 2 more and a chance for Overwatch to drop yet another. And that's even if they caught you (how?).
You can spend those points on whatever you like, am I telling you to not take serpents? No, I am saying don't spam them and rely on that save to make you lazy on positioning.
No, you're saying in your previous posts that they are "a liability and will lose you the game" against armies that ignore cover or have fast assault elements. Word for word. That pretty much translates into "don't take them" in my view. If you meant to say "write balanced lists" or "play well" then I would have 100% agreed with you from the start, but that's not what you said.