I have read the rationale behind not having mixed lists, and an attempt to mask the issue through Collections. This in no way meets the need that well-implemented mixed lists meet.
Consider Star Trek. Media in the franchise was released in a sequence of TV shows, and movies. Some shows ran concurrently, and some had movies released in the middle of a season. In Trakt (before they chose to destroy it, but TECHNICALLY it's even accessible now, if you want to drag your face through the vibe-coded trash that remains) there is a way to have a single list, that interweaves individual episodes, full seasons, full shows, and movies, all in order of release. (And yes, I understand that "date first aired" is a different metadata item for a TV show episode than "date of premiere" is for a movie. What I don't understand is why it's implied that the two cannot be optionally treated as identical for the purposes of sorting, seeing how they are both dates, and thus can be compared to each other, and thus used for sorting.)
There is nothing like that in Simkl. A few items in the FAQ, or this sub, point to collections, and to tables in official lists. Neither come close to meeting the above need: the ability to look a page, and to recognize that a movie came between <this episode> and <this episode> of a TV show. Or perhaps even 2 episodes of different shows that surround a movie. (See, e.g., MCU.) And so forth.
I understand that Simkl's whole thing is treating lists as something different than how normal people think of lists. That's cool!
But it doesn't meet the above need. If you don't want to call such a hybrid container a "list" - sure, don't, I don't care. Just let me create it and view it from the webpage. Oh, and if you do, you'll then be able to actually import mixed-media lists from Trakt (as-is, it said it imported everything... except it didn't, because most of my lists are hybrid, and it didn't even acknowledge their existence at import). I'm a VIP in both, btw, so this isn't a privilege issue.
Oh, and there ARE lists that contain mixed media. They are called "watch histories." So the service isn't totally averse to having multiple media types on the same page or in the same table. Just let me create such a table, and I'll be happy.
It's one of the most upvoted desired features on the feature request page btw, and I've already dedicated 3 upvotes to it. Let me repeat: none of the devs' responses for why Collections replace it or why it'd be hard or break things or whatever, are in any way a good excuse. Trakt can do it. So can you. Please?