Does a 'router like' board need HDMI? Well different people, different opinions on that. Problem is when you start to mix up those things, openwrt may have some needs to for a kernel which 'we' don't care (I'm not really familiar with openWRT). How do you ensure that your device gets regular updates (kernel and userspace side) when you've a combination whit doesn't fit to both distributions? This may need additional work to keep your device updated. For me, such router/network boards should run with a (patched) mainline/vanilla kernel or at least with a kernel which isn't EOL (the only device which doesn't get regular updates in my home is the outdated iPad and the android phone, sony decided that my phone isn't worth to get updates a few months after I bought it ).
I am just giving an example, of how when I mixed them something that did not work is now working. Also, I could find the battery level which was not available when I used pure openwrt. The main kernel is openwrt now, so when I write on the terminal is appears as openwrt.
And yes, my software is built on openwrt. Another advantage is that they have the LUCI interface ready which is a GUI of configuring the router. That is a problem. I agree mixing between two different OS is not a good idea. But I will try and see if I did the opposite,