Considered “harder” by whom? Windows enthusiasts have certainly considered Windows to be more sophisticated since the first public version of NT, and more secure by XP SP2, at the very latest.
Windows has been harder to attack than OS X since Vista. I don't think that's changed--Apple is generally slow to correctly implement security features that require kernel changes compared to either the Linux devs or Microsoft.
If I remember correctly, OSX didn't have a good ASLR implementation until 10.9 or 10.10, and has generally lagged behind on OS-level security updates and exploit mitigation (NX pages, ASLR, etc.). It was routinely the first thing hit in the early Pwn2Own competitions (partly because exploits on it were a breeze, partly because the laptop was usually a little nicer and you only got the laptop back then, too--but also because moving from crash to reliable exploit was easier).
I haven't followed security updates in Linux in the past few years as closely, so I can't comment on it as knowledgeably. From what I remember, it was about on par with Windows for difficulty to attack in Vista/7, but there's been a lot of security features that Windows has added since then. From what I remember, a fully hardened Linux box was just as bad to attack as a hardened Windows box, but Windows has generally required less configuration to get that right. (I think SELinux still isn't enabled by default in some distros?)
theqruq had this in one of his slides. I'll try to find the link.
sorry I do not know the meaning of shilling in this context, English is not my native language.
https://grugq.github.io/presentations/COMSEC%20beyond%20encr...