Every OS has vulnerabilities. Whether those vulnerabilities are actually exploited in the real world is another matter. If you practice safe computing, it is extremely unlikely your Mac will ever be compromised.
IMO probably the biggest weakness is the fact that Apple provided a way to reset the admin password buy simply booting to the Recovery partition. Read about it here. So everybody thinking their data is secured by using a login ID is operating under a false sense of security. If you want a password to protect your data on a Mac you need to turn on Filevault, which is very secure.
No one OS is secure and no one OS is secure by default. Also UNIX is not immune for viruses but we are lucky because there are not so many or they don't writing them.
Firewall (pf from OpenBSD) has so many options not just what Apple give you as default and you never now how secure are commercial operating systems...