Signal's subpoenas have always left a sour taste in my mouth. I just can't believe that they are getting so few, at least some cases they'll just send the standard letter out and will try to get something. Having no list of how many they have rejected would at least increase my confidence in them a bit.
But the bigger issue is, that they data they provide is just too good to be true for the majority of users. Signal has a push token for the vast majority of accounts otherwise they wouldn't be able to send out push notifications on iOS and would waste at least some battery on devices with Google Play services installed. The subpoenas always seem to affect people who have an Android phone without Google Play services installed. In my eyes is too strange of a coincidence to be true.
Signal does at least a bad job of explaining what kind of data they keep on an average user.