Levelup monsters seem better about it as far as I've seen so far but it comes down to the need to make room for magic items. It wouldn't matter if players started with 20/18/17/16/15/14 if the monster math had a baked in allocation of magic items expected and a sidebar somewhere that said words to the effect of "if you are running a campaign with few or no magic items you should consider giving players bonuses such as these at x y & z levels" but lacking that kind of baked in math adding them requires shifting numbers on the fly with every monster in session after session. The new numbers are quite low or sporting a severe deficit as some have noted specifically to create significant room for magic items rather than just putting off the problem to when players start looking to upgrade their +1 doodad.
This... doesn't make a lot of sense.
For starters, by having such low stats, you're penalizing not just combat stats (because it can be improved by magic items) but also the stats used for exploration and social. Since your system basically allows for a person to be good at one thing
only, you are forcing the players to choose one of those three tiers to be OK at while making them bad at the other two tiers. Even in 05e, which never emphasized exploration and social as much as they thought they did, this wouldn't be cool. And it's definitely not cool in LU. Your system is basically crippling the PCs two-thirds of the time.
Secondly, you don't need to "shift numbers of the fly with every monster." You can plan in advance. Or you can let some battles be tougher or easier. Remind your players that both using tactics beyond "I attack" and retreat are options.
Third, you still don't
need magic items to defeat the monsters. I'll have to go reread the MM but I don't think
any of them require magic to kill; magic just makes it easier and faster. And if there are monsters that
do require magic, then you can either introduce nonmagical ways to defeat them such as bane poisons or traps, or you can simply not use them.
Fourth, many players, possibly most, don't want to rely on magic items to be cool for them.
They want to be able to do the cool stuff. That's one of the advantages of LU, because the maneuvers allow for much more cool stuff in combat. Likewise, not every player is going to want to build a stronghold just to get a +1 to a stat, because not every PC has a stronghold type of personality.