Not really caring about it, but testing it a bit, these are my observations:
- Delay of the infrastructure, at least for ADS-B is quite small, 1-2 seconds (compared directly ADS-B received aircraft position to the one received by SafeSky. For OGN/Flarm it doesn't seem to be noticeably more
- Arbitrary additional delay can of course come from the mobile network connection
- "I remember times when the provider cut your connection when they found out you were using internet in flight" - but that's a thing of the past, i.e. not relevant for the discussion
- "No, do not use a source with unknown age of information in flight." This is not a valid point either, since there is a simple solution for it. It is called a timestamp. SafeSky does know the age of a received traffic message (and could show a warning and/or extrapolate its position. Don't know if it does though, or how old data can be to be "acceptable").
At least when flying lower, I tend to get a decent internet connection with acceptable latency in Germany - higher up it's gone of course.
I guess a direct receive is still preferable whenever possible, but it's not like it doesn't work at all. And the direct receivers have their limitations, too (e.g. no Mode-S only traffic).