It is clear you don't understand how it works and that it does in fact work for published monsters. I have made hundreds, if not thousands of 5e monsters. I have checked the CR on probably an equal number of monsters in the published books by WotC and 3PP and fan creations. I know what I am talking about. I've done it, I've used it, and it works. I will have to agree to disagree with you at this point.
It is clear you don't understand the problems. That they are a miserable failure for new DMs - and that
new DMs are the people who most need tools.
There is a two-page lists of traits and their cost in the DMG as part of the monster building guidelines. Again, if you didn't realize that I am not surprised you think they don't work!
I'd forgotten that but this is correct. As is the fact that about half of these traits don't adjust the CR. And also (as a pet hate) these aren't actually traits - these are pointers to traits that mean you need to cross-reference the monster manual.
You can get that with the monster guidelines as is. Just trade those massive hit points for more damage. Again, it seems you fundamentally don't understand how it works.
And this is the basic problem. If you
have already mastered the system you can get good results. If you just read the system and try and do the obvious things it gives you complete crap. They are not user friendly and do not work out of the box for the most basic examples.
A system that gives you terrible monsters unless you actively master and push the system
is terrible when it comes to the single most important thing - making things easy for newbies to learn to DM because there is a lot to learn.
I'm strictly talking about the guidelines not the monster design itself. The 4e monster guidelines gave no guidance on how traits, conditions, etc. effect the level of the monster. That was very frustrating. I made a lot of 4e monsters with the monster builder and the books. I know what I am talking about.
Meanwhile every single monster you made in 4e both had a role and had at least a decent balance once they fixed the maths in the MM3. There was no equivalent to the awful CR 1/2 monster I posted that is not just RAW but actively using the default options.
Why I said it is trickier at lower CR it is because you are dealing with fractions (1/8, 1/4, & 1/2 CR) and people have a hard time with fractions.
Which means that it is absolutely necessary that they should work when the offence and the defence are both kept in balance. They don't. They fail hard.
They still work when used properly. I will say the monster guidelines are very robust and do take some time to understand so I agree that can be difficult for new DMs.
They only work properly when you push the systems
far away from the default at low levels. They actively give you a strongly negative play experience if you leave the settings anywhere near the defaults at low levels. Which means that for any sort of tentative DM who is trying to run their first campaign they are quite literally worse than useless, actively leading to bad games.
Even the 3.X monster design rules did a better job at teaching newbies and making it easy for them to give good results.
Now it might be that all that needs fixing is rebalancing the offence and defence below about CR 2. But this is the single most important section of these rules for the single most important function of the DMG. And that the rules are a completely miserable failure at the most basic and most important functions means that yes I haven't delved into them more deeply.