Treecats also seems like a good reason to go animated. Obviously, they can be done as CGI inserts into live action, but given how much screen time they get in series as a whole it would be nice to not have to deal with all of the "CGI character" issues you get when using that approach.

From a logistical standpoint, the fact that so many characters fade in and out of importance in different books makes for a casting nightmare if you want to follow the books closely in terms of which specific characters are along on which specific missions. "Ok, we don't need you on this year's show because your character is assigned to a different ship during this plot arc, but please don't take a recurring role in anything else during this year that we're not paying you, because we'll totally need you again next year when we do this other arc" is a hard sell for actual humans trying to make their acting careers work. (Movie schedules work somewhat differently, and big-name actors that primarily do movies can be booked during specific blocks of time since they then schedule other movies in the other blocks of time, but tv actors tend to be needed for an ongoing chunk of time if they have a regular role on a series since more shooting time is involved. A limited series can get around this somewhat, but the constant coming and going of Harrington-verse characters still seems like it'd be really hard to manage if you cared about getting the details right.)