I’m an adult on a pony. I’m petite, and size wise, she is a perfect fit for me. We compete at 2’ and 2’6", where there is a mix of of horses and ponies, and I don’t feel like we have been penalized at all. We do pony strides at all the shows, although she is a large pony and has a large stride, so we could get horse strides without her flying around.
Like I said, right now, at lower levels, there is a mix of horses and ponies and it’s fine - it is mostly kids on large ponies not ready for the 2’9" pony class or other beginner adults. At some point, I think we will move up to 2’9" and I may feel like we are at more of a disadvantage there as I think we will be the only adult/pony combination in that class. If you’re in a class where there are other ponies, then I think you would be at less of a disadvantage.
The question about made pony vs green horse really depends on the rider.
Being an adult is a drawback to competing h/j on a pony, but I was aware of this disadvantage when I bought her. I just don’t show enough to really care, especially when I consider this horse an otherwise perfect match.