I definitely remember a distinct decline between the first and second series. The second series and all subsequent books I read (I haven't read all of them, I'm not a millionaire) weren't bad per se, and I did kinda enjoy them, but... the plot of the first series was engaging and the characters were generally well-rounded and well-written. The further series didn't seem to have that same depth or intelligence - not to mention that the stories and even turns of phrase and extended metaphors seemed to become repetitive.
i think it was somewhere in the power of three or something. The first part was the best into the wild, I came back to reading the book last year and i was shocked at how awful the quality actually was. I dont rember the exact book but i think it was the power of three. Sadly i think maybe some of this could possibly be the distributes problem, sometimes big time publishers have to force authors to create more books. Or give a due date, and thats very stressfull
This is a complex question, but I've spent a lot of time thinking about it and this is the best way I can answer. I've read the first 2 books in the first series recently just to see if it would hold up. I discovered that the problems present in the third series and beyond are essentially the same as those present in the first series just on a bigger scale. To be honest the Erins, as a group at least, aren't the best writers. The first series I believe was the only one planned ahead of time (the second might have been). This allowed for them to correct any issues and a tied together continuity better, have the plot make sense, have a decent sense of pacing and better emotional moments. However the quality of their ideas has largely remained the same over the years. This is most apparent when you look at their romance subplots, a quality of which is one of the few things that has remained largely consistent over the years as well as their pension for well written descriptors and fights.
Warriors was never the best, but I think the quality began to drop after the first series. After the Erins were forced to continue a series they weren't prepared the write.