The biggest on-line complaint, right now, is about invisible drivers on track. The indicator of a problem is that, while waiting in the pits, some drivers may not have coloured backgrounds for their list number, on the left of the list. It should be filled in with either blue or green.
The best way of dealing with this, until PDI fix it, is to wait in the pits until everyone syncs up with everyone else. This can take some time. I once saw it take almost two minutes per driver, so be patient. Therefore, using a timed start would not be advisable, under those conditions. In my experience with 1.15, this seems to work best.
There seems to be some help by drivers who can't see each other to leave the lobby and return, but that may also mean that syncing needs to restart again.
PDI has been made aware of this issue and I am assured that they will deal with it.