I think the pacing issue as a reason for 5e being hard to GM carries a lot of weight. It's something I struggled with personally when I first started running 5e and it's something I've personally seen newer 5e DMs struggle with in a way that makes the game less fun.
Simply put 5e is built around the assumption that there will be more encounters per long rest than most groups can reasonably get done in a session. So what happens is generally either:
A. The party does fewer fights per long rest than the game's assumptions call for, which does bad things to class balance, refocuses the whole game on nova tactics and constant long drawn out brutal fights (to counter the nova tactics), and worsens a lot of the flaws of 5e that already exist. From the comments I've seen from 5e newbs this happens a lot and causes a lot of problems.
B. The party has more than one session per long rest. With a party of adults with constantly changing schedules this is a pain in the ass as you have to track partially-spent resources across sessions and have naughty word like the party getting attritioned down in the middle of the dungeon and then BAM! a fully rested cleric pops up among them since Bob couldn't make last session but could make this one and you can't tell him to wait until the party's out of the dungeon before rejoining the game since Bob is your friend and he's bringing pizza.
These problems can be overcome with good DMing, but I found them to be a big pain in the ass personally.
This same problem was even worse in 3.5e (the infamous ten minute adventuring day) but not really an issue in TSR-D&D (due to combat being so fast and PCs having fewer resources) or 4e (since 4e works fine if you have just 1-2 big fights per long rest). I think that the playstyle of a lot of 5e newbies would really be muuuuuuuch better served by 4e than by 5e.
My workaround for all of this was "put the PCs on a boat" (or space ship) and only allow the PCs to get a long rest in port (not on a random desert island). They had to go on long sea journeys between each long rest so I got to really grind them down Oregon Trail-style which was fun. It also explained why Bob's PC wasn't around last session but is here now (he was busy doing naughty word below decks on the boat). And why Jim's character suddenly isn't around during Jim's vacation (he got sick and is in the infirmary below decks, we have to get him to port for treatment!).
But I've played in whole campaigns that constantly ran into these kinds of issues such as waaaaaaaaaaaaaaay too few fights per long rest or people coming back to the table after a few week break due to real life and then scratching their heads and trying to remember if they had used Channel Divinity or not last session since the DM wasn't enforcing any centralized method of resource tracking from session to session.
I think this is based on a fundamental misunderstanding of what pacing is, and how it is managed within a TTRPG. Pacing is, at it's core, a response to the human attention span. Human's have an attention span of roughly 9 seconds. They can choose to refocus that attention on the same thing, repeatedly, for, on average, around 5 hours. These are just general guidelines, of course.
Pacing is the DM incentivizing players to continually refocus their attention on the game being played, over their phone or other distractions. Almost every case of a bored player is caused by a pacing error.
Often you see pacing described as high and low beats. This is a simplfied version of the concept. You do, generally, want to alternate between high and low beats, but that is not the extent of pacing and only doing that can leave other glaring errors. Tension building is another aspect of pacing, you see this in novels and how they are structured, but that can be ignored here. It's not really relevant to the point.
A common example of a pacing error is the over narration of a scene. Going into depth, and describing every detail, is almost always wrong. If your narration exceeds around 9 seconds, you are asking the players to refocus on the same narration. Instead your pacing will be better if you narrate the basics and leave the specifics to questions from the players. The reason this is the case, is its easier to refocus on the answer to a question you asked than to maintain focus on a continued narration. In essense, the players are more invested in their question than your monologue.
This can be extended into the idea of buckets, each bucket is a player. And you put chips representing time in those buckets. You put these chips in these buckets whenever that player is being engaged directly, when they have the spotlight. Anytime the DM is narrating, you are only putting those chips in the DM's bucket. A player that goes too long without a chip being added to their bucket, will lose interest and become bored.
The entire point of my rambling is that pacing is independant of game mechanics. You can use mechanical aspects of the game to facilitate aspects of pacing. But those mechanics themselves are irrelevant beyond their ability to fill that role. Your example of "pacing" issues within 5e's mechanics are simply not pacing issues at all. They are perceived game balance issues, and are completely unrelated.
EDIT: I could have addressed why this also doesn't apply to the rhythm of the story aspect of pacing. But I can do that later if needed. In short, it's largely the same reasoning. The rhythm of the story is largely disconnected from mechanical interactions.