Yeah, I think they are a more formalized version of some things that people did in 4e (both in some of the official stat blocks and less formally). So it is cool that they basically said "this is good, we will make it a regular part of the system, so then designers will know to always do it." I always thought that was an element that 4e brought to D&D design that was good. Like role and power source and monster role/type and the structure of powers kind of gives everyone a solid idea of how to design something.
So, in 3.x my perception is a LOT of things are basically 'duds', or at best stuff that has only very niche applicability. Like a lot of classes, even ones in the PHB, are drastically under-powered, and others over-powered. Same with monsters, a lot of them vary greatly from the CR you would expect, only work in certain situations, or are super deadly in certain situations or combinations. That rarely happens in 4e. It seems to happen fairly rarely in 5e either, and I think all of the various guidelines help. Though, as you observe, overall encounter design in 5e is tricky.
Honestly, every implementation of D&D gets a few things wrong too. 4e would give you a perfectly FUNCTIONAL encounter of level 5 if you put 5 fifth level standard orcs into it, and you could follow a template to say which specific mix of stat blocks to use, and it would play out mechanically. Put that encounter in a fairly mundane static location though, and don't supply any interesting plot elements, and 4e produces something that is OFTEN, maybe USUALLY dull as dust to play. I mean, it may be fun once, but the SECOND such room will have the players off to the kitchen for sure! This is easily solvable, but the books don't really tell you how (they hint, but they're far from pushing their point enough, and they are too mild in their advice).
So, 5e's encounters are not so much of a problem that way (well, they tend to play out a bit faster, or at least you can do stuff more often). OTOH the CR system gets a little wonky, and they seem to be less interested in tactical situations and thus mixes of monsters and clear roles for each monster.
Ah well, maybe 6e will be the God Edition!