It is the sort of thing that might be worth discussing down t' pub but somehow I can't quite bring myself to be bothered to turn my thoughts into deathless prose. But..
(mild spoilers ahoy but so mild I'm not going to put a cut)
1. It seems to me that the rivers don't quite fit in with the rest of the mythos, like Ben A. had various ideas that don't quite fit together. On one hand there are the rivers, and on the other, all the other magic stuff (spells, ghosts, vampires (normal and jazz) etc.). The tone seems different.
2. There seems to me to be a definite sense of magic inflation as the series continues - more and more magic, more and more magic humans, beings etc. and more and more people in on it. It is possibly an arbitrary personal preference but things ostensibly in our world (except for the magic / whatever) should to my mind should not have the entire population of a medium sized country in on the magic / whatever.
3. As I have said before, it is hard to read any contemporary detective novel written in Britain without feeling that the writer had between 1 and 1.99 eyes on its adaption for television. Not surprisingly, given that Aaronovitch wrote for TV (some Dr Who and Jupiter Moon, of all things), he is at the 1.99 end of things. Not necessarily a bad thing, but the novels are very much in the manner of an ongoing TV series i.e. there is a main plot for each 'episode' but quite a lot of side plots coming and going and cutting across books. This works for TV where the next episode is in a week's time, I'm not sure it is terribly satisfying when the next episode will be coming out in a year's time - IWBW (If we believe Wikipedia) the fourth book is 'Broken Homes'.