I use sandboxed Google Play Services, Signal has been fine... I cannot say it's always immediate (I'm not always on my phone :-) ), but I have never been frustrated for "missing" a notification. Like when I check my phone, the Signal messages are there, they don't appear 3 days later.
Oh, they certainly don't appear days later. But sometimes they do between ~1-30 minutes. This is a well-known issue, you can find many topic about it. When you open the app, everything comes in all at onec.
It is sometimes annoying to me. I cycle a lot and sometimes my wife or somebody else sends a message when I'm on the go (like picking up some extra groceries) and then I miss it.
There is a longer discussion with various people here:
