RE: "What is the upside to Trakt over Letterboxd tho"

I'm glad you asked:
-Trakt covers a larger variety of programming to log.  Despite pulling from the same sources, Letterboxd often does not have certain films, or will nitpick what to include. I've been told that they've relaxed this and now allow YouTube content to count as Cinema, but when I joined, it wasn't an option. In addition to film, you can log/rate/comment/collect on individual episodes of Programs and Serials.
-Accessibility.  LetterBoxd's apps have a few design flaws, some buttons don't scale and it looks unreadable in a greyscale display mode
-profile design leaves a lot to be desired imo. (personally) I don't enjoy reading other people's letterboxd profiles, the whitespace commitment is too harsh
and you aren't given any options to really make it your own in terms of looks/content.
-api, letterboxd has been teasing their api since launch, and even made public statements saying that pro supporters would also be funding the further development of the api. fast forward, still no api.  Trakt however, has an well documented and oft maintained and open api, that you can use today. Trakt is really good about taking the conceit of data in aggregate and expanding on it.  I've logged everything I watched into it and if I ask trakt "what Comedies did I watch in 2021?" it knows and will willingly volunteer that information.  I can turn it into an RSS Feed, export it, lots of stuff. Trakt is the program for the Chronic Watcher inside all of us.

one thing I will concede Letterboxd has over Trakt is their Editorial dept.  Letterboxd articles do often turn me on to new cinema that I've never heard of before, or Directors that will eventually explode into popularity, before they get ultra popular. I'm sure Trakt could just as easily achieve a bespoke curation for it's userbase,
but for now they're spending that money where is truly matters on making features that work and work well.

if youy want to see these features in action, please consider taking a look at my profile, though currently not feature complete(I need to  make more lists), makes significant use of all of the basic features in some way or another
https://trakt.tv/users/julianmyjulian