Ratatouille is definitely a good ride, and will be a great compliment to the World Showcase. I've not done Mystic Manor or Pooh so I can't really make the comparisons between them, but if you take the ride on its own merit, it is a fun and impressive ride. Yes it's not particularly ground-breaking, but there are some clever features that do make it that bit special. Another point that while it does rely on screens for portions of the ride, I think at least these fit in fairly well and include sections that would be extremely difficult to replicate within physical sets, also the large scale of the screens help keep the illusion that you have been shrunk down to the size of a rat.
Regardless, I'm more excited for Guardians as it is an original ride, so I'm sure it's going to be an impressive concept whatever it turns out to be, but unfortunately I think I'm going to miss it on our next planned trip in 2020 along with most of the other announced additions.