To be fair, it's gotten quite a bit better in recent iOS versions than it used to be, but I'll agree that it's still not nearly as bulletproof as Reminders is. Keeping a widget on your Home Screen does help it sync more often, for example.
In my case, though, I'm less concerned about notifications. I use a few, but mostly the way I work with Things is to have it on hand to check my list of what needs to be done for the day, and I open it several times a day for that purpose. I also don't bother with notifications on my iPad or Mac, as I always get them on my Apple Watch, so anything else is redundant.
I've tried this setup a few times, most recently this past spring. I could never quite make it work in a way that didn't feel like it was adding clutter to my life. Still, I love the concept, and I
wanted to like it... I still have another 10 months left on my latest Agenda purchase, so I may give it another try.
Ultimately, though, this is the thing about productivity apps and systems — and why there are so many. Each person has very different needs. I was once a hardcore OmniFocus user, but I found that to be overkill, but as much as I keep trying to come back to Reminders, it just doesn't work for me once I throw dozens of tasks into it — and I've tried various approaches at organizing my lists, from contexts to areas of responsibility to simple time intervals (something I came across in
a great YouTube video series by Carl Pullein).
That last one seemed to work the best for me — especially now that there are tags and smart lists in iOS 15 / macOS 12. I also keep thinking that the Agenda integration will be the secret sauce that ties everything together, but I'm really more of a task-oriented person than a notes-oriented one, and trying to drive my workdays from Agenda feels like more effort than it's worth.
Just my two cents, of course. To each their own, and like I said, everybody has different needs, which is why there's never a one-size-fits-all solution. At least three of four times a year I put Things on the back burner and try other apps and approaches, but I always seem to keep coming back to Things, so while it's not perfect, I guess it's still the best tool for my needs.