I had a look at Substack’s RSS feed and I see what Manton probably doesn’t like, they embed every bit of text in a CDATA block, which is total overkill. All you have to do is encode the left angle brackets and ampersands in text and it’s fine.
That’s the feed for my blog on Substack, just for testing.
If you view source on that in the browser, you’ll see the problem.