Just watched the first season. Tbh to me it seems like Rescue Riders is predominantly a kids' show and the main audience is quite obviously not adults (Unlike the HTTYD movies, Riders/Defenders of Berk and Race to the Edge), so imo I don't really mind the low polygon models and talking dragons, and my young nephews and nieces don't mind them too.
I have been a fan of the HTTYD movies and tv series for a long time, but if you want to criticise Rescue Riders, you really have to think with the perspective of a MUCH younger audience. If you're an adult, you'd have to understand this show was not made for you.
It uses the HTTYD universe but you definitely have to suspend a lot of disbelief if you're used to the normal HTTYD. It's set in another village presumably far away from Berk. But that being said, it's pretty enjoyable in my opinion. The plot isn't half bad and they introduce new dragons from the book frequently. The dragons having a certain number of shots rule is still in place too. And you get used to the talking dragons, which only the protagonist brother and sister can understand.
Unlike Paw Patrol which is also aimed at very young children, I didn't feel like my brain cells were being killed off within the first few minutes. But, to each their own. However I believe that it means that both the young target audience AND older audiences /can/ enjoy it.