This was so much not my experience. 3e was a huge breath of fresh air where we could just play without constantly having to have rules discussions. The rules just worked.
LOL wow.
That is literally the diametrical opposite to my experience of 3.XE, with multiple different DMs. We absolutely constantly had to have rules discussions because of the "a rule for everything" effect, and the fact that many rules piled on top of each other. Stuff that in 2E would have been disposed of as a called shot, so just an attack at -4 would often require multiple rolls, some at penalties, and we constantly had to look up whether a certain attack or whatever had pre-existing rules, and what they were.
Combine that with AoOs, grid-based movement, vastly more complex arrays of situational bonuses and penalties, combined with tons more stacking buffs, and the rules got in the way just insanely more than 2E.
I'd like to say there were fewer ambiguous rules in 3.XE than 2E, but that wasn't true. As a proportion of the total rules, 3.XE had fewer ambiguous rules, but it had so many more rules than 2E that you bumped into ambiguous ones just as often.
We saw combats that'd have been resolved in 15-30 minutes in 2E take 2+ hours in 3.XE, even after playing for years. Especially if anyone wanted to try anything fun or clever.
4E started off better than 3.XE because they ditched "a rule for everything" and went with a much smaller number of more general rules, combined with a lot of exception-based design for class abilities, which meant far, far less checking rules, and far more looking at the character sheet and immediately seeing the info. 4E also largely killed buff-stacking and significantly reduced situation modifiers. But as you went up levels, more and more Reactions, Immediate Actions, and Interrupts got involved in proceedings, combined with increasingly complex abilities/magic items on the PC side, and more complex monsters on the DM side, and it slowed down to not quite 3.XE levels of grind, but significantly slower than was fun (whereas at lower levels it was a lot of fun).
A poorly organized book, which still has valid information, does not make the game difficult to DM.
D&D is inherently more demanding to DM than most RPGs, because it more heavily relies on DM interpretation by design than most RPGs (which has positive and negative effects), and also asks for significantly more prep work, and more adherence to certain concepts than most RPGs (like in encounter-building).
So at a baseline, if we grade RPGs on how hard they are to DM, D&D is already one of the ones which demands making more calls at the table, and doing more work before the table, hence probably one of the "harder" ones.
Add to that that the 5E DMG was both exceptionally poorly organised (even compared to older editions), and very, very lacking on the "Good advice on how to DM", especially for a newer DM, and you're creating a situation where one of the harder to DM RPGs has been made harder still to DM. Plus the DMG was featured a lot of largely irrelevant waffle (admittedly not uncommon in DM books for various games), which I think must have helped confuse people a bit re: what D&D was about.
Does that make the game "difficult to DM"? I don't know, because difficulty is relative. Is any RPG "difficult" to DM? D&D is more demanding than most, that's for sure. But I would say given how many people taught themselves to DM, I don't think D&D was "difficult to DM" in an objective sense, like vertical free-climbing is difficult, say. I do think WotC made it harder than it had to be. The book wasn't just poorly-organised, it was fundamentally lacking as a DMG. It's probably the worst DMG D&D has ever had (4E's being probably the best, interestingly). And it was likely that way because it was rushed out, like all of 5E 2014 was.
Fortunately there are live streams, blogs and podcasts galore to help people out if they need it.
I would say these were a mixed bag, because finding the wrong ones could be very off-putting (in the sense that that level of DMing seemed unachievable) or not actually help at all because they don't really offer relevant, basic advice. Further, a lot of the info was pretty contradictory and often presented in quite prescriptive ways. There was absolutely gold out there, albeit not really much in 2014/2015, but you had to either get lucky, get a good recommendation, or sift through a lot of dross-you-wouldn't-know-was-dross to find it.
I do think that these existed probably significantly lessened the impact of the 5E DMG being terrible though, so overall a positive for sure, but WotC didn't support them in any way, so essentially they got lucky after putting out a pretty rubbish DMG. Certainly just being able to watch D&D being played helps people who have never seen it.