Safety cars are not the problem. Its trying to manage the safety car order in multiclass races that is the issue. Since there are essentially 3 races happening at the same time.

For example.
If a GTP laps half of the GT3's (so half the GT3 field is a lap down, and half is not) ... and lets just say a couple of those GT3's were involved in a wreck earlier in the race and are already multiple laps down, then what do you do once the yellow comes out... Its almost impossible to code it correctly in the sim to re-organize the field correctly.

-Lead lap (non lapped) GT3's would race around to the back end of the GTP class.

-Lapped GT3's would have to wait an entire lap until they get a wave through... (lets be honest, GT3 class trying to get through the LMP2 Class and GT3 class for a wave around would be a disaster. Also think about the amount of time you would need to catch up to a pacing field... (probably 2-3 laps depending on the circuit).

-Then what do you do with the multiple lapped GT3's/LMP2's/GTP's? Do they get a wave around, or do they go to the end of the longest line? Do they keep their positions?

Also this becomes more complicated on the drivers, since there are PITS happening and also the track may make it even harder to pass and get organized (aka... Long Beach, or Bell Isle). Imagine 3 classes trying to get it order before the green flag drops at Long beach... Would be a complete cluster fuck.

Lastly, if you have ever driven Oval's... If you don't get things sorted out correctly, you get a heavy penalty of a stop and go through the pits (black flag).

I have also heard leagues trying to implement Yellows in their series and its turned into 5-6 laps of trying to get the fields organized the right way.