With modern CGI I don't think they would have any problems with the tree-cats.
I suspect some of the main stoppers would be size of the main story arc and the changing emphasis. The overall story arc bringing in the Mesa and Sol would, I suspect, be a bit over long/slow for a tv series. They would want some sort of conclusion at the end of each season and I'm not sure the HH series would offer that. Also the series has gradually moved from primarily action to primarily political. Whilst I think Weber can just about get away with the change of emphasis in book form I'm not sure they would like it for a tv series; they would want the whole thing to be one or the other.