I kind of agree with you on the encounter building rules, especially in the 2014 version. I do not reference CR at all, I find it both incomprehensible and useless and prefer the XPO calculation method.
I wish that that method worked better. It worked extremely well in 4e--not perfect, especially if you used the rules to dynamically scale a monster to a new level--but extremely well in general.
I genuinely think (based on my experience) that after third level it should be hard to accidently TPK a party. I have no idea what is going on in your games.
I mean, the long and short of it is that these GMs presumed PCs functionally took almost no damage, no matter what or how many monsters they faced, and (in at least two cases) presumed that PCs did not really need to rest other than Long Rests.
You did say that you warned some of the DMs, what were your warnings and what where the signs that alarmed you?
The two I knew personally, I attempted to warn very early, as in after like the first session or so, in part because this was very nearly their time GMing any TTRPG ever. (Their very first was a group they were concurrently running games for, had started a couple weeks before my group.) I don't remember the exact words, but it was something to the effect of:
"Hey, I know you're a new GM and such, so I'd just like to note that low level characters in 5th edition can be very, very fragile. That doesn't mean you should coddle folks, but with so many new players and you yourself being a new GM, please try to be careful. The dice can be extremely swingy, which means death is a big risk. You might consider starting us at a higher level, to avoid this problem."
The first GM brushed it off like it was nothing, blithely threw whatever sounded good at the party, and then couldn't understand how our group had TPK'd when his other group had beaten his expectations. I wasn't exactly happy about his kind of brusque dismissal of my concerns, but figured hey, he's the GM, it's his call. The TPK came three encounters later, and the very next encounter after my warning had nearly killed literally every single party member, as in, everyone was down to
low single-digit HP with no Hit Dice remaining
and my character had a disease that was
guaranteed to kill him because he couldn't make the saving throw to survive it. (GM fiat let us survive that first situation, which the GM also found impossible to believe that this group should struggle where his other group didn't.)
I gave essentially the same warning, just augmented with "Hey, remember <Bob>'s game? Yeah this is exactly what killed that game, so please, please be careful, I trust that you have good intentions but I've already seen it go wrong once with a friend of ours." He was not completely dismissive the way the previous GM was, but he was kinda condescending and basically said something equivalent to "don't worry, I'm good for it." We then had a TPK two sessions later when his planned dungeon caused the party to split into two groups which each got wiped out by separate encounters. We didn't have any choice about the party split, it was literally "the floor gives way and two of you fall in" type stuff.
The third warning was to someone who was a stranger to me. I approached it less casually, since I didn't know the GM like I did the previous two, and tried to keep it as circumspect and limited as possible, basically just, "Hey, I've played in two campaigns already that broke down because of a low-level TPK, so I'm really hoping to avoid seeing that again. Please consider how fragile low-level characters are." I was met with an answer so venomous, it bordered on "Shut the hell up or I'm kicking you out of my game." It was not that explicit, but it was definitely said with a threatening edge, an implication of "if you dare question me again, I
will kick you out". So I kept my peace, and lo and behold, the exact thing I had tried to warn about happened.
After that, I stopped bothering with any kind of warning. If it doesn't work with friends, and it invites such hostility from strangers, it's not worth risking my neck. Just keep my head down, hope I get lucky. I never did, of course, but I did at least try.
