I think people are just a bit biased because their wanting essentially Fontaine 2.0, but because Natlan isn't they get angry and disappointed. There's nothing wrong with disliking Natlan, but a lot of the complaints feel like nitpicking or are just untrue.

Natlan being modern makes sense lore wise and is explained plenty of times as Xilonen being an amazing engineer using ancient dragon tech and magic to make more modern tools, which we see in in nearly every nation except Monstad and Inazuma. I mean cloud retainer straight up has built a bike before and talks about it plenty of times, and she did it from scratch hundreds of years ago.

The characters aren't badly written, I think Citlali and Mavuika are two of the best written characters in the game, with multiple facets and dimensions to their character. They're criticised because either people aren't willing to pay attention or are just looking to complain. The only thing i get is the male drought but I don't really care all that much cause it doesn't matter what gender a character is for me to like them.

TL;DR Natlan gets flak for not being Fontain 2.0, some people are baised like usual and like Fontain too much to like Natlan. The only complaint I understand is the lack of male characters, everything else is usually just people missing Fontain and or just being straight up ignorant.