Complaint that's more or less specific to science fiction and fantasy:
Cultures don't scale that much. I can see there being a tribe/group of people who do X (live in a big city, are good at mining, like wearing fluffy dresses, etc.) but that doesn't mean I can buy a whole PLANET of city-living miners in fluffy dresses exist! Leaving aside the obvious issues with your entire planet being urban (the amount and cost of space traffic necessary to bring in food for that huge a population would far outstrip any planet's wealth or resources), history has shown us time and time again that human populations tend to form splinter groups when the population gets big enough to split. Try to show me a "vacation planet" and I'll ask you where all the workers live - do they own expensive resort real estate too, or do they have a hidden slum outside of town? Same with "this race of aliens is nomadic and really good with riding horses" - logic would dictate a mix of rural and urban living anywhere the same as we have here. Often alien races end up being shorthand for specific stereotypes (warlike, greedy, noble, etc.) and authors/scriptwriters completely ignore the fact that any race would have a huge range of normal variation in individual personalities.
Pretty much this! I have a copy of a Star Wars book called "The Guide to Alien SPecies" or something like that. While it's a fun read, your complaint sums it up exactly! For example, one species might be described as highly xenophobic. The whole species. Really? All members of the species hate outsiders for whatever reason? (Superiority complex, religious reasons, fear that contact with others could lead to colonization or contamination of their "pure" culture) Even if a planet's governing body holds those beliefs, surely there would be dissenters? Surely there would be differences of opinion, with different groups having different ideas, or positions at various places along the ideological spectrum? For example- the governing body's position is one of isolationism, but in reality, citizens have different ideas- some are very isolationist or xenophobic, others hold a moderate position advocating perhaps just enough contact with outsiders to avoid becoming out of touch, or coming up on the wrong end of a technology gap, and others still (even if they're a minority and catch flak for it) advocating for openness.
I'm trying to avoid this in my own writing- true, I'm dealing with a race of beings who coexist with humans on this planet, who have their own culture and ideas. But at the same time, they're not all ideological clones- The majority believe in a symbiotic relationship
with humans, but there's a minority who have been badly treated by humans, and don't want to play nice anymore. In some cases, they've integrated peacefully with humans, two cultures coexisting in relative peace. In other areas, they tend to butt heads.