I've written various things in English (papers, thesis, abstracts, etc.) and I loathed using that language every step along the way. English is such a clunky language for communication, especially in science.

-It has loads and loads of words, many of them overlapping with subtle differences that go over the head of many non-native speakers yet still carry some kind of difference between intended and perceived meaning. Some of them are just synonymous, such as Latin/Germanic cognate couples (godly vs. divine, tiredness vs. fatigue, etc.), and you just sort of have to learn them all if you want to understand everything you read or hear. So you apply yourself and it turns out that you're still not understood because the vast majority of the community doesn't speak English as a first language either and also has trouble understanding one or the other form you're using.

-Its pronunciation and spelling is completely wild and inconsistent. You can't guess how to pronounce a word you first encounter. And in scientific and technical presentations there are a lot of such words! This doesn't even take into account the various accents within native English speakers (I'm somehow supposed to be able to understand both the Californian and Australian accents no matter how nerve-rattling they sound) and non-native speakers (who also have the same trouble pronouncing new words as I do, except they'll mis-pronounce it differently than I would).

-The way sentence structure is designed favors the apposition of many different words without any preposition, leading to ambiguities. This is of course exacerbated in papers with absurd word count constraints that must somehow fill the needs of print journals despite the fact that 99% of people read them online. It also makes for very terse, opaque and generally tedious reading. The only thing more annoying than reading a paper is writing one, because you're also going to have to write in the same opaque style in order to squeeze as much information as you can in order to fit your 3 year project's worth of discoveries into a 1500 word Nature letter.

-It's unfair that non-native speakers be discriminated on peer-review based on the quality of their English. Reviewers and editors like to pretend they're unbiased but they're not. The quality of the work is independent on the author's ability to write in a foreign language. I'll readily admit that judging someone based on their mastery of a language is not unique to English speakers but this particular situation is especially egregious since most native English speakers do not, in fact, speak any foreign language.

-It's generally a constant reminder of the cultural hegemony held by the US and Anglo-Saxon countries in general. Native speakers are advantaged since they have to learn less and are still less discriminated; scientists flock to Anglo-Saxon universities despite other ones being as competent if not more, but less well-known; the top journals such as Nature or Science are either British or American, reinforcing this bias. Univerisities ratings are often based on publications in these Anglo-Saxon journals, further deepening the bias. And lastly, Anglo-Saxons in general (and Americans in particular) have a very strong Not Invented Here syndrome whereby anything that was discovered outside the anglosphere is disregarded or met with much more skepticism than usual. Every so often an American paper comes out touting a new innovative "amazing" method that will completely revolutionize a field despite the principles having been discovered years before but outside the land of the free. It's frankly annoying.