I personally think it's one of the better seasons. While its writing style clearly differs from what we saw before, it gave us some great character development and amazing episodes - even some of the best in the entire series, such as Brewster's Greatest Gift and Chugging Home for the Holidays. The songs were such a fun addition too. To me, S6 overall definitely felt like a return to form after S4 and S5, going back to a more chill version of Chuggington that just focuses on the chuggers and their relationships with each other.
As for the character removals, which are the reason I guess many people dislike S6: those were necessary to stay in the relatively strict budget Herschend had for the season. It's also worth remembering that S6 was released after a 5-year hiatus; it needed to reintroduce Chuggington's world and characters to a whole new generation of kids, and it definitely wouldn't have been possible to introduce ~40 chuggers all at once. I mean, even S4 and S5 already had no time to focus on most of their oversized cast at all.
Also, I cannot really understand why some people hate on Tai and Rosa; let's finally face it, Calley and Skylar got sidelined hard in the S4/S5 era and would've been cut from S6 even if the newbies didn't exist. I don't necessarily see anything wrong with their models being reused either - lots of the same type of locomotive are produced in real life too. This is also something we've seen established in Chuggington before with the Harrison and Dunbar recolors in the original seasons. Also, again, budget.
