IMHO, the biggest disadvantage to OpenVPN is that it's not interoperable with the vast majority of products from "big name" network vendors out there. Cisco & Juniper's security and router products don't support it - they only support IPsec and proprietary SSL VPNs. Palo Alto, Fortinet, Check Point, etc. don't support it, either. So, if your organization / enterprise wants to setup a site-to-site extranet VPN to another company and you've only got an OpenVPN appliance, you're probably going to be out of luck.
That being said, some network hardware & software companies are starting to embrace OpenVPN. MikroTik is one of them. It's been supported since RouterOS 3.x:
Also, for the longest time the only way to run an OpenVPN client on Apple's iOS required jailbreaking. This is not so, anymore:
Overall, the situation is improving. However, without vendors like Cisco & Juniper implementing it in their products, I can't see large enterprises adopting it without facing interoperability problems.