Probably practice in getting around Japanese novel format aspects such as changing tenses between past and present (English normally uses past tense uniformly for fiction), the fact dialogue lines are kept on the same line with the speaker being stated only right after, the fact dialogue and internal thoughts aren't always distinctly separated out, etc. These are all things I'd encountered myself as a reader without worrying too much about it, but being faced with the task of restructuring it so it'd be readable to English speakers and not confusing was a very different experience. I feel like I've definitely improved a lot in handling this between the Kizuna novel (the first one I ever tried) and the Ex-Aid one (the most recent one), but I've got a long way to go.

For something more specific, I realized very intimately how much Ex-Aid likes to reference existing memes in Japanese gaming culture and how much it demands familiarity with it to be able to translate for it.