For a start, setting up entry restrictions (which characterise GT Life mode) is nearly impossible. Rather than choosing particular maximum specifications or drive train types, for example, if you want a restricted race you have to manually choose the cars that are legal every single time.
The problem is they're based on your own garage, meaning if your mate has a mid-engined sports car and you don't own it, they can't drive it. The alternative is unrestricted races, where everyone turns up in their most lunatic ride and then spears off at the first corner.
It doesn't help that anyone can hit the Start Race button, and probably will given that it's placed right next to the Free Run button, kick starting the race and locking you out of selecting a new car. We pity anyone who tries to start a private race without a Bluetooth headset and very patient, co-operative friends.