I'm going a bit far off of my wheelbase here, because exploit development and mitigation is not an area I have a lot of experience in, but the sense I've seen from security researchers is that OpenBSD's security posture is far more mixed than their developers' touting implies. So while OpenBSD may have some features that are well-regarded (say, their malloc implementation), they can do a shockingly bad job in other areas (such as ROP mitigations).