The things that you traditionally cook slow and low are things that are traditionally cooked to death. Often, that means tough, collagen-filled cuts of meat. The reason that these are cooked "low" is that, because they are braises the meat is in a moist cooking environment and using a hotter oven wouldn't bring any advantage. Your oven might be at 500F, but the meat in your braise isn't going to get above 212F if it's submerged in liquid. Pressure cookers allow you to cook in a moist cooking environment but at a higher temp than you could in an oven (or anywhere else at normal atmospheric pressure. Because the water gets so much hotter when you're pressure cooking, foods break down much more quickly than they do when using a slow cooker or doing an oven braise. Both cooking methods are (often) about breaking down collagen using heat and time; it's just that the time is longer and the heat lower when you're doing a traditional braise.
Apart from the time and temp difference, the key difference between an oven-braise and a PC version is that there's no evaporation/reduction in the pressure cooker, and the top of the meat above the liquid doesn't get as crusty (if that's what you're into). That also goes for exposed veg, which won't get as brown.
A way to improve the flavor of PC braises is to strain them after cooking and reduce the cooking liquid by half to simulate the effect of long evaporation. That's usually what I do. I brown the meat, add stock, pressure cook, strain, and then reduce the liquid. In the meantime, I roast vegetables in the oven. When everything's ready, I toss the meat together with the sauce and the vegetables and hold it at "keep warm" for a while to let the flavors come together.
