Taking the whole software/communication/hardware complexity into account, imagine, how many bugs and vulnerabilities may be present in such non-open-source syst

Taking the whole software/communication/hardware complexity into account, imagine, how many bugs and vulnerabilities may be present in such non-open-source systems? Based on the recent history where we know even 0-day phone exploits exist, how dangerous would it be if a vehicle (or most of them simultaneously) can become being controlled remotely?