I did the 5$ patreon tier for it & don't regret it after looking over the mount stuff & skimming through a few areas.
@AdmundfortGeographer did a nice little summary
earlier that I agree with. The mount stuff is definitely interesting & I might use or draw from them for magebred mounts if I ever run o5e again. With the rest of the system so stripped down for simplicity there isn't much room for them to build mount rules around but they did a good job of it. Right now I'm most interested in seeing what areas of rules & crunch they carve into over the next few issues because it looks like arcadia will be filling a niche sorely lacking in 5e.
Back when I ran 3.5 & earlier I didn't use or allow everything from dragon, but it was an option to consider or starting point that gm & player could use to cook up something fitting together often enough to be a big help. It seems like this will fill a similar useful niche & the price is low enough I won't feel bad if an occasional issue doesn't seem useful to me.