This is wham-bam-thankyou-ma'am classic space-opera style SciFi. Ignoring the unsparkling prose and some minor quibbles, it is one of the classics of the genre and mostly survives this my first re-reading since my youth.
The characters are all stereotypes and the social setting about as advanced as in Asimov's "Foundation" series (though there's a plausible excuse for that: society has collapsed, and rebuilt itself). The spaceships are run like WWII battleships, or possibly more like Napoleonic era ships.
But no-one who cares about that kind of thing should be reading this kind of book in the first place; space-opera SciFi is the place for ideas, not subtle characterisation, and this does have a nice setup and nice aliens.
But having said that, let's play the fun game of what's wrong with the ideas.
The first and usual, but excusable, one is that the motley crowd of characters gets rushed off to the job, instead of people sitting down carefully and working out what to do. Secondly, I think it is odd how little attempt at stealth they make, and how quick they are to engage with the aliens. Thirdly, there's their lack of caution, and the improbable competence / intelligence of the aliens, but I'm descending into trivia now; let me claw my way back into the light.
I think it unlikely that a solar-system wide civilisation would collapse all together at once; the advantage of being un-collapsed when everyone else has reverted to barbarism is too high, people would scheme to be in that position. More likely would be waves or patterns of collapse.
I think the sketched alien civilisation is too "flat": having individual Masters as the top level, and nothing beneath it, would be unlikely to work. Coalitions would be necessary, but then the alien shock at people who both look up and down would make no sense.