I’ve owned several different Australian saddles over the years. I love them. They’re comfortable and secure. I think they’re great for trail riding. I certainly wouldn’t use one for jumping, aside from hopping over logs on the trail. I’ve got a horse now that has a big trot. Posting properly in the Aussie saddle on him is difficult. Getting into two-point to jump would be nearly impossible - the saddle is designed to keep you from pitching forward and keep your seat in the saddle, not to facilitate your getting up out of it into two point.
Yes, most of them put you in a chair seat because that’s precisely how they are designed. See, e.g. this youtube video:
https://www.youtube.com/watch?v=dIL0srrA238
I don’t use that exaggerated chair seat with my feet sticking out in front, though.
I’ve got two in my tack room right now. One is the more traditional style with panels like an English saddle, like this one:
https://www.downunderweb.com/lite-rider-australian-saddle/
One is built more like a western saddle, like this one:
https://www.downunderweb.com/stock-fender-australian-saddle/
I don’t know that one is better than the other, although if you have a wide, round horse, it’s probably easier to get a good fit with the western type.
As far as purchasing one goes, the same rules apply as to any saddle purchase. The cheap ones on ebay are crap and you get what you pay for.