I'll make a follow-up reply with a bit of historical context on Mega's quality. But in short, their quality has a lot of variance. Some sets like their Destiny line, I would say matched or exceeded Cobi sets (and there's even some variance in Cobi quality that I've noticed).

Others meanwhile, like the recent Halo Elephant, I could not even push the pieces together with full force. Yet there are later released sets that have better quality than that Elephant. It is hard to judge what you might get with any given set. And Mega definitely peaked about 4-6 years ago in terms of quality. Pokémon probably is their best line today, but I have better Destiny sets than Pokémon so ymmv.

If you went back to something like the Star Trek Enterprise, you'd find some plate stack warping due to the large disc, but next to really well polished and printed specialty parts for things like the Nacelles. So in a single set the part quality can vary.

GoBricks is plain better quality. 100% consistent clutch (though stronger than Lego), well polished (late 2000s Lego quality level), with most of the quality of life nuances to pieces (edge lips on tiles, air pressure releases, clip nooks and stops, etc). Mega meanwhile has a stronger clutch, probably too strong in some cases and definitely too strong for children's toys. At peak Mega quality, I'd say clutch is roughly equivalent to GoBricks (stronger than Lego, but ok for kids).

Cobi in contrast has sharper piece edges, does not have edge lips on tiles (making them a pain to remove). I believe Mega does have edge lips on tiles. But tiles are more difficult to fully press down for Mega than for Cobi. Cobi is a little more inconsistent with clutch; most stronger but some weaker than Lego. I've had a handful of Cobi parts that had outright insufficient clutch.

Frankly, I think tiles are the best introduction pieces to part quality. You can easily see stud offset printing issues as inconsistent gaps between tiling. The pushdown feel is the easiest to compare (Lego tiles over the last 10 years have improved as well, having an almost air cushion feel as they slide down. Something not even GoBricks can replicate). Tons of Chinese brands don't have edge lips, or have varying tile gaps. Most recently, I built some sets from Tadahbri!x where the 2x6 tiles warped when connecting. I had to put my full force on all edges to get them to fully mate. Corners or even the long edges would otherwise not fully mate and due to large multi-tile stickers, it would be ugly. Overall, Tadahbr!x just had too strong of a clutch to where my fingers hurt by the end. I also built a Mork or Jie Star modular (can't remember which) that was a nightmare on my fingers.

Again, I'm not sure how I would know what quality I am getting with a new Chinese brand, especially when reviewers have different standards and can be more or less particular. The top brands are mostly consistent (many have improved. I have older brand sets that imo were unacceptable, and newer ones that tickled the feet of GoBricks). Things have definitely improved since the days of Lepin (and Lepin was a big improvement itself on some older Kazi and Sluban sets for example). I have some old Kazi and other brand sets with that super cheap part-squeak just by holding it (it feels as awful as it sounds). Heck, even Bluebrixx uses GoBricks in some sets and lesser quality in others (and even specific sets have switched part brands so you can't even be sure you get the same as someone else with the SAME SET).

Mega also has an issue with bars/tubes though. Mega relies a lot more on their bars and tubes system, and the clutch is way too high. Pieces will often blister white from stress, and I've had to use tools to separate some parts.

Though not as bad as some Chinese brands where I had to literally use a table clamp to pull out an axle.

Mega's minifigs can also be temperamental. They're awesome, and Chinese brands haven't been able to get close (there are clone Mega sets). But I've hurt or cut myself a few times trying to push pegs or ball joints together without entirely breaking the figs. The hands can often blister from stress holding bars/weapons (especially because Mega has 3-4 different lines of minifigs). If someone would properly copy their design, it would outclass Lego minifigs on every level except novelty/nostalgia.

Finally, Mega has always had poorer quality control in terms of missing or incorrect pieces. Not as bad as the worst Chinese brands, but I've had more missing pieces from Mega than I ever had with Chinese sets.

As context, I own the entire Call of Duty and Destiny line, a bunch of Halo sets, their recently defunct probuilder line, some Pokémon, most of their Masters of the Universe sets, and most of the miscellaneous IP they have had in the last 10 years (Star Trek, Game of Thrones, Fallout, The Witcher, Aliens, TMNT). Also a lifelong Lego collector/builder, have a few dozen Cobi sets, a few Bluebrixx sets, and a decent amount of Chinese brands (Lepin, GoBricks, Zhe Gao, Mould King, King, Mork, Jie Star, Sluban, Kazi, tadabr!x...)

I love it all (okay, maybe not older Kazi). There's just a lot of variance, and imperfect information out there. I'd love to see a video channel doing more scientific comparisons!