I suspect meme hit on most of his frustration:
And if you are pasting plain text copied from a text file or PDF, I recommend pasting it into Book View. If you paste it into Code View it'll be a mess because Code View is meant for valid HTML code.
And because it is not valid HTML it will not let you save it.
But I don't know what you can do about that. Even if you will be able to save non-valid HTML in the next update, someone who pasted junk into the HTML window isn't going to be able to do much with it.
It is sleek and very functional tool right now that never has problems unless I mix up the change case and the h4 buttons (operator headspace).
The best way to approach using Sigil for the first time is to start with something small and not essential: kick the tires, rev the engine and push all the buttons, right click everywhere and see what happens.