If considering Windows only environment, what's the advantage of introducing OpenVPN as the company VPN service, instead of Windows built-in protocols? Especially the new SSTP protocol already overcome the one of the weakness of PPTP, which may not go over firewall/NAT.

I'm wondering is there any reason not to use Windows integrated solution. The strength of the security can be an issue but I'm not sure how different they are (I know MS VPN was vulnerable but is it still?)

Thanks.