Just a few thoughts to help combat the complaints about backdooring incursion sentries, without removing the tactic entirely.
First, add the sentries Shield to the UI, along with their health. There are so many false positives, and while those should be addressed separately, displaying the current shield level seems useful for many reasons; gauging a real back door attempt vs false positives is certainly one.
Secondly, minimap display. Anyone who attacks a sentry should automatically display on the minimap for 3(?) seconds (refreshing with each attack)
This would still let people try and back-door a sentry, while giving teams the information they need to respond appropriately.