Garretts are boost referenced, not a vacuum pot....just as a completely external gate would be.
A spring holds it shut, boost overpowers the spring at the set level and opens the gate. Holset uses the same scheme.
The problem with an integrated wastegate is primarily the exhaust rejoin.
Some get it right, others don't.
The BW EFR series looks like they got it pretty much right with the wastegate, but they've done quite a few "experimental" type things in that series turbo. That's why I've been eyeballing one for a while. I'm just not ready to pony up the kind of dough they want. Got a few contacts inside that say it will work great and they'll size one for me, I just don't have the money that they want for one.
Maybe some day......like when I win the lotto.....
External gates aren't the last word either. Very few manufacturers size a "non wastegated" turbine section to effectively deal with an external wastegate.
A non wastegated turbo is typically sized to pass all of what the compressor makes through the turbine. A wastegated turbo takes into account the hole the wastegate create and sizes the turbine for that.
It's a very tricky balancing act and there's a lot more involved to it than just popping an external wastgate in the piping pre-turbine on a non wastegated model.....