As someone who has run quite a few 3E, Pathfinder, and 4E D&D games and only a handful of 5E D&D games, I have to say that running large numbers of encounters in a 'workday' is problematic for two reasons:
1) A lot of players get very disengaged by their character going 'I Fire Bolt it.' 'I Fire Bolt it.' 'I Fire Bolt it.' repeatedly because they have to budget their abilities.
Less of a problem the more & more varied encounter/short-rest abilities each character has. A party of Warlocks, Battlemasters, and Monks would ameliorate that problem, for instance...
...though I guess the lack of healing could be an issue.
There are a lot of players who legit don't mind describing how they're swinging their sword after taking the Attack action ten rounds in a row
I've heard that, too - more often I see players who are just happy to spam a high-damage attack and get excited about high rolls & crits.
2) If things go south and the players end up blowing a lot of their abilities in the first one or two encounters, they feel like you're picking on them as you drag their characters through the rest of the gauntlet. This is also true if the players are casual or new to the group and aren't in tune with how D&D structures workdays. They aren't going to be sympathetic to your rejoinder of 'that's just bad luck/you not getting the rhythm of the game; better budget your abilities better next time'
That can be an issue, sure. But to the extent the game is /designed/ to work on a specific pacing, in order for encounters to be challenging and classes to at least spot-light balance, at least theoretically, you can't just let the party that overplays it's first encounter or two off the hook, you need to hammer them the remainder of the day to make them regret that choice so they won't do it again. If you fall into the 5MWD rut, you'll be making over-leveled encounters to 'challenge' the party, who will pull out all the stops to win, and then be forced to rest again. Aside from the class imbalances that style brings to the surface, the level of challenge presented by encounters becomes increasingly brittle, as the party 'novas' to annihilate the day's encounter makes it seem 'too easy' and by the time your crank it up enough to appear challenging, you set up the potential for a TPK of the anticipated nova doesn't quite go off (someone blows a roll or misses their cue, and suddenly PCs are dropping).
I also think that 6-8 encounters does violence to the narrative of action-adventure fiction (since it's a D&D-specific trope that doesn't have genre or metafictional justification) that can only be justified as a gameplay/story tradeoff
It's not like it's the only thing about D&D that runs starkly counter to genre.
but that's a separate discussion altogether. Just speaking from a gameplay perspective, it disengages certain kinds of players and I'm getting rather tired of boards like these treating such players as powergamers or n00bs.
That's the least of the problem, IMHO. It's the kind of issue that can end up giving a prospective new player a poor first experience with the game.
Look, I've played a lot of 5E D&D. I like the game a lot. I played since it first came out and I play a combination of home, online, and AL games an average of 8 hours every week.
I have, however, not DMed a lot of 5E D&D. That's there for full disclosure.
It is very different from the other side of the screen. DM Empowerment is a very real thing, and you can use it to run the best game you can, or for some other agenda...
I DM'd AD&D (1e & 2e both) for a long time, and running 5e comes easily because of that hard-won experience. I doubt 3e/4e DMing experience would translate quite as well. But one of the things about the classic game was an expectations of characters managing their precious daily resources, in order to clear/explore as much of the dungeon as possible, because it was prettymuch a given that when you walked away from it to rest, treasure was going to get up and walk away and new monsters move in, or some rival adventurers were going to white tornado the place while you were gone. 'Gold rush economy' was about more than just inflation.
I've never used the encounter building guidelines in the DMG until just recently. One of the things that surprised me was that I overran my daily XP budget front loading the adventuring day with one hard and one deadly encounter. Basically, the math provides you with 6 MEDIUM encounters... +1 easy encounter, if you really want to hit your max budget
Keep in mind that the budget is for exp the party gains, but that the difficult modifier for being outnumbered doesn't add to exp earned, just to difficulty. So there's a fairly large amount of wiggle-room, there.
We've heard all of this before. All those reports of 5E being the most popular version ever? Fake news.
To be fair, 5e took 3+ years to move the kind of units that TSR was doing at the height of the 80s fad, each year.