Here's the thing. If you have a system with a large difference between strong and weak saves, you create a system where some characters usually succeed and some characters usually fail. That's a problem if that doesn't match your expectation of what high-level heroes SHOULD do.
So, define your expectation. Then see if the system matches it. If it doesn't, then change it.
Most of the arguments in these forums happen because people define their expectations implicitly. Lay out what you expect to see happen, right up front, instead.
Fine. I thought it was manifest from the math, but I guess not.
My expectation is that a single AoE spell or poisoness gas or whatever in the first round should not take out 3 PCs for half to all of an encounter out of a party of 5 at high level. Especially if the PCs taken out are the spell casters, so there are few ways to break them out of the enchantment, poison, stun, hold, whatever. This is a TPK waiting to happen because the 2 remaining PCs do not typically have the resources to both fight foes and help their allies. Encounters are balanced for 5 PCs in this example by a DM, not 2.
This should not happen at any level, let alone high levels when the PCs really should all be heroes.
Taking a couple of PCs out for a couple of rounds, fine. But when the DC is 20 and most 18th level PCs have a +0 to +2 save, that's ludicrous and bad design. IMO.
It's more ludcrous that at high level, even moderate saves of DC 15 to 17 might be required every other encounter.
Yes, the PCs could have magic items. Yes, there are some PC abilities that might boost abilities. The fact remains, these factors, if relevant to the situation, may or may not be there, especially since Concentration is on a one spell at a time basis.
And to tell you the truth, just due to how dice can go cold or hot, I would suspect that an ambushing set of monsters that are considered medium to hard, but not deadly, could easily TPK a mid-level party partially due to the weak save factor. Given the nature of this, it might be a miracle for any group to survive past level 12 or so unless the DM takes steps to protect the PCs.
In 6 months, people will be talking about this like they did for weak saves in 4E. The difference is that there were usually 1 out of 3 weak saves in 4E and there are 4 out of 6 weak saves here (2 out of 3 major, and 2 out of three minor). The odds do not look good. Eventually, I expect an errata or a splat book will have ways to mitigate it. Not quite sure why the designers did not see this coming and if they did see it coming, wow.
