I've only tuned with mitsubishi OEM internal gates, so YMMV, but the actuators are where the magic happens. We run our OEM, stock, internal wastegates well into the mid 30psi realm. The actuator is what will limit the max boost achievable, and also affect spoolup.
I've never put an internal sytem on to a previously external, so I don't feel safe giving an opinion. But we can run an external gate with OEM boost control measures (but with new settings). The design of your exhaust setup will likely be very important switching to internal. In silly motorsports like autox, people in a class where changing stock boost parameters in the OEM ECU is illegal, they modify the exhaust in a way that it causes the wastegate to create boost creep, increasing power.
I never have seen one of our internal gates fail. And only heard of very few actuators going, not seeing it myself.