L2TP doesn't traverse NATs and other network firewalls very well. Which isn't relevant if you yourself control all of the hardware between endpoints (e.g. point2point) but if you plan on using it on public networks (e.g. Starbucks WiFi) then expect to run into it being blocked sometimes.

OpenVPN was designed for the "Starbucks WiFi" scenario, it can traverse a NAT, and uses standard TLS. It therefore appears like any other HTTPS traffic and will be very infrequently blocked.

There's nothing inherently wrong with L2TP/IPSec; it is secure and fast. Just nicer to blend in with other TLS traffic for reliability reasons.

For a specific example, GoGo Internet (in-flight WiFi), doesn't allow L2TP/IPsec unless you use UDP encapsulation. OpenVPN works out of the box.

I would be interested to hear this as well. I favor L2TP/IPsec as it's built into iOS and MacOS and it was easy to configure my edgerouter-x to be a server for use on the go.