Tesla programmed its cars to go very slowly through a stop sign rather
than completely stop, in very
limited circumstances, much as many
drivers do.
This is regarded as "unsafe", but given the special conditions
required for the car to do it, I think that officials are exaggerating
that danger.
I know of two real reasons why the Tesla car is unsafe.
The "full self-driving" feature is unsafe because it's based on the
pretense that the driver will pay full attention to the driving all
the time, just as one does while actually driving. Lots of drivers
will say they will do so, then not do it.
The computer systems are not safe for drivers because they contain
nonfree software, and that is always an injustice to the user.