I've been running BandHelper on an iPad Pro with various iOS 11 beta versions and with the release versions of 11.0 and 11.1. Here are the outstanding problems I've seen:
1) Changing songs or navigating to a different page while audio is playing can leave the audio playing, or crash the app. Workaround: stop the audio before changing songs or navigating to a different page, or turn on Help > Utilities > Use Old Audio Engine. FIXED in app version 3.2.4.
2) Even if you stop a tempo before starting another tempo, it might play the old tempo for a few bars before updating. Workaround: turn on Help > Utilities > Use Old Audio Engine. FIXED in app version 3.2.4.
3) Stopping the audio on some devices will crash the app. Workaround: turn on Help > Utilities > Use Old Audio Engine. FIXED in app version 3.2.9.
4) If you're using the new two-column document display with multi-page documents, the next/previous page functions won't jump precisely to each page. Workaround: none at this time. FIXED in app version 3.2.4.
5) If you're using the iCal feed in a Pro account to display events in the iOS Calendar app, the login to the feed will fail. Workaround: use the Open URL link on the Settings > Schedule Sharing page, which doesn't require a login. FIXED in iOS 11.2.
I'll update this thread if any other problems are reported.
In general, I'd recommend waiting until iOS 11.0.1 or 11.0.2 is released, to avoid any early iOS bugs. I'd also recommend not installing iOS 11 if you have a device that's more than three years old. I always hear of performance issues and instability when running new iOS versions on older devices. In this case, that would mean anything older than an iPad Air might not perform well with iOS 11. Remember that you cannot downgrade your iOS version if you upgrade and don't like the results.
Feel free to share your own experiences here.