Many disadvantages to the new Reminders (as of iOS 13) vs. Things 3. Here are three:
Archiving Projects
If you choose to use Reminders lists as projects, when you're finished there's no way to archive them. This is ridiculous. If you use Reminders, you essentially have to let old projects/lists pile up forever, or else you have to delete them completely. OK, fine, let's assume I'm willing to delete a project/list once it's finished. The completed reminders will still exist in the void without a list for context, right? Wrong. What's even more ridiculous is that deleting a project/list deletes all the reminders inside of it, including completed reminders. How insane is that?
Compare this to Things, which understands that the historical record matters to people. Finished with a project? No problem, there's a handy archive button.
In this way, Reminders doesn't seem like it's designed with projects in mind. Rather, it seems that Apple assumes that you're going to have a number of fixed lists, or contexts, that won't really change. That's fine for GTD but terrible for organizing projects into their own lists.
Tags
Lack of tags is a big deal. On Reminders there are no tags. This means you have to manually add tags by typing into the notes section or somewhere else. This is an OK hack, but it's sucks down the road if you decide you'd like to rename a tag. Are you really going to go back through hundreds of previous tasks and edit the "tags" you added into your reminders? Not likely. There's a reason tags exist, and the fact that Apple still hasn't added this functionality is pretty shameful.
Things 3 is way ahead in this department. Not only are there tags, but you can nest tags, which is super powerful. Unlike some apps with nesting tag functionality, Things 3 has a dead-simple interface that makes it really nice.
Search
Search in Reminders is terrible. For example, as of this writing, there is no way to tell the reminders app to only show incomplete reminders in search results. I wish I were kidding, but I'm not. I have scoured the app incessantly trying to find this and it just doesn't exist. If Apple wanted to keep things minimal, the obvious thing to do would be to simply have the drag down on search results to disclose some hidden option to "hide completed," or something like that. Or just add a setting in the Settings app. But no such luck. The problem with this is that it means when you're searching for an incomplete task, you will often see completed tasks appear in search results above the incomplete tasks. Is this some kind of joke? Am I really supposed to wade through dozens of completed tasks from years ago, scrolling down exhaustedly until I finally find the brand-new incomplete task I was looking for? Hell no.
Compare this to Things 3, which was designed by people who actually know how to build software. By default, search ignores completed tasks. If you'd like to search across even completed tasks, simply tap the "continue search" button and bam: you'll see completed tasks as well. Thanks Things 3! No thanks to Apple, the trillion-and-a-half dollar company that doesn't know how to build search into a basic task manager.
Conclusion
These are just three really big ways in which the Reminders app falls on its face compared to Things 3. Yes, Reminders does have some advantages, in some cases meaningful ones, but these aren't minor issues I've mentioned. Search is vital. Tags are fantastic. And forcing people to delete projects when finished is silly.