External wastegates have a few advantages over internal ones but the main one is they allow for better control of boost pressure on high boost applications. They do this in two ways; first they prevent boost creep, basically the internal wastegate port is too small for lots of flow so after a while it will choke (can't get any more flow through it) this means more flow goes through the turbine and pushes up the boost produced by the compressor. Secondly they help prevent boost fluctuations and boost spikes, again the small diameter of the port/penny valve means that they are slow to respond to increases in boost pressure (e.g. as you come on boost) causing an over shoot in the boost level. In worst case this can lead to oscillations as the wastegate struggles to achieve the desire boost pressure.
For most GTiR’s there’s very little point in an external wg and most of the above can be achieved by porting the wastegate port.
As for chatter its best thought of as "Transient surge" and is a compressor phenomenon that has next to nothing to do with wastegates. I f you look at a compressor map, under normal operation (on a well matched turbo) you will be somewhere around the middle of the map. When the throttle body (/ies) close the boost pressure is still maintained as the turbo is still spinning very fast due to the inertial of the rotor system, but the flow decreases; this moves you to the left of the comp’ map and into the surge region.
The flow separates from the compressor blades (stall in the aerodynamic sense nothing to do with the turbo not spinning) and the compressor operation becomes unstable, leading to audible pulses or “chatter”.