Did you experiment at high level with rules as written? If so, how did that work out for your group?
Yes we did, my memory is a little hazy but I remember the following things:
- The saving throws were not too much of a problem, because as written the monsters have such low DCs
- Rather than non-proficient people being terrible at them, proficient people were damn near unassailable.
- At high levels, the consequences of failing a save are considerably worse than at lower levels. A 40% chance to fail at low levels means you might be in trouble, at high levels it means you are dead(ish) 4/10 times.
- Monsters resisting PCs was a problem, PCs got too many save-or-suck/die spells off.
- We quickly found that DCs and To-Hit on the monsters as written were too low. Once we changed them up a little to be similar to PCs, the proficient people were OK, proficient with a good stat were very hard to hit, but non-proficient people were getting laid to waste.
- The 4e style defender roll become much more important when you have monsters that cannot really hurt some of the party, but will demolish others. For instance an Ancient Medusa had to have the tank marking it, as the Wizard had very little chance of making his Con save if he got targetted.
- Further to that point, as saves get siloed into being great or terrible, monsters become a bit rock-paper-scissors. A creature that attacks vs Str save will devestate the back rank and not touch the front rank. Likewise with a charming creature, keep your front rank the hell away while the bard and wizard sort them out.
- Paladins with a Holy Avenger rule, as they should. Granting most of the party Adv on spell saves is awesome.
- At high levels, you can always have a spell prepared to use against the lowest save. So can NPCs.
All in all, I think it is possible that the system can work passably as is, if the monsters have very well determined DCs. I am kind of OK with the tough Barbarian only having a 5% chance of being turned to stone and the wizard having a 50% chance of avoiding it. It is not balanced for PvP at all, but I don't think that needs to be a major consideration.
The universal proficiency + advantage change was well recieved by my group, but we were starting at high level, it would have been awkward to suddenly introduce that at a particular level where the maths fix was needed.