Arguably, the easiest thing to resolve is identifying an operating system that has proper support for OpenVPN. If the tun or tap device is not supported, it quickly rules out OpenVPN or limits the specific features of OpenVPN, but is often overlooked. Both iOS (all versions as of this writing) and Android (also, all versions as of this writing) do not support the tap device.
There are other operating systems that don't support virtualized network devices at all. FreeBSD jails, for example, don't support the tun or tap devices without some significant configuration and startup tricks. Many embedded operating systems, generally on routers and switches, do not fully support OpenVPN. Even if your platform of choice claims to support the virtual network devices, it's best to do some digging to ensure that OpenVPN runs reliably.
