Well I tried it with both the IX-D2 and the XVIII.
I have the Direct2Drive version (I'm from Canada).
Balkon Gerat hydrophone works 100%. Picks up contacts surfaced, sumberged, snorkeling or not snorkeling. Can go to the hydrophone station and use the hydrophones manually. I also tried the default hydrophone the IX-D2 sub comes with in the 1944 career, can't remember which one it is, but it worked too.
Silent running also works, whether you are snorkelling or not. I use the hotkey and didn't try the interface button. I was periscope depth.
The only problem I can see is the CO
2 bug while snorkelling. You can snorkel for hundreds of miles, do a quick surface, then go right back to snorkelling again.
My only wish is getting a radar-detector mounted on top of the snorkel
.