I binged it all week and got to the finished it about an hour ago. Dot is my all time favorite character in this series, as well as her Pokémon; every episode featuring Dot is a superior episode. I love that Horizons' story felt like, "Stop the evil drug dealing corporation from selling the evil Pokémon empowerment drugs all over the world". I think Terrastrilaization is cool, along with mega evolution.
My new favorite Pokémon is Pecharant and Mochi Mayhem is my absolute favorite episode from Horizons
What I didn't like is that I wasn't emotionally attached to any of the villains... except maybe Coral, but overall not really. I'm a huge villain fan.
All and all, I don't really have any downsides or criticisms of Horizons. I may in the future, but I really liked it. 9.9/10 We need more Dot episodes (by we I mean me... idk about anyone else).