Most all ponies in the Fizzy mould tend to be quite substantial. We used to use them to deter boys who liked throwing ponies on the roof and they were very effective 'lances' in those days...
My childhood Masquerade has not got the thin plastic problem so maybe it's batch. I have had others with that soft plastic feel but my Masquerade has always been a sturdy character. Good thing too as I used to throw her across the playground to make her fly...
I don't think she has either of the other problems either but I do know that most of this set are prone to both. I also wonder whether it is something more seen in HK made ponies and ponies of the early years as there do seem some ponies more prone than others - or whether it relates to batch and how the plastic was formed at the time of manufacture. I wonder this because there are Starshines that go totally brown and icky and those that don't...