UDP protocol is useless if nothing is done to prevent fragmentation.
Otherwise, it is at best unefficient.
There exist routers on the internet that filter fragmented packets with specific
data. This is hard to debug because TCP connections hang randomly when there is
no OpenVPN encryption.
Now, only TCP is enabled by default. A second protocol should be there for
better performance when possible, either existing UDP one (provided it is
guaranteed there is no fragmentation) or something better (GRE ?).