Garbage in, garbage out. If your EPG xml does not have the information that Channels needs to populate good guide items, then it's just not going to work.
Channels has a set requirement of metadata and we can't and won't work around random EPG files to make everything found on the internet work. It's just not tenable. Instead we'll provide what is needed so that you can ensure the XML data you are using works best with Channels.
If a source has guide data that is close to working but needs some data massaging, that's up to the person that's using the data for their custom channels. Small projects like scripts that read in the EPG XML that then add in extra attributes that make it work better with Channels is what is going to be required for a lot of things. It might be outside of the comfort zone for some people, but not for others.
As @tmm1 suggested, the pluto-for-channels project is a great example to check out to see how it does its best at providing data that Channels can consume to get best results in the guide (and for recordings). But this is a separately operated open sources project, and not officially supported by Channels. Again, it's a great example to use if you want to help take an EPG source and get it in great shape for Channels.
We could do better at documenting things like what @tmm1 mentioned above and we'll work on it. We want to make sure we provide the right tools and documentation for you guys to create projects like these to extend Channels and make it amazing for yourself and others.
UPDATE:
I've added some Proptips to the Custom Channels documentation to show how to make the best of Channels by tweaking your EPG XML.