Fiona Collins writes in her intro that she is primarily a storyteller, and you can certainly tell when you go through this short collection of stories.
Written as they are, these would actually make good bedtime tales to read to kids, and so I'll keep Ceffylau to hand for when my nephew's a bit older - but if you're an adult and know the folktale of Rhiannon (for example) and read Ms Collins's version of it here, I can see why one wouldn't grok with this book. I'm not familiar with all the stories, but the ones about Rhiannon and King March are sanitised to the point where even Disney would raise an eyebrow.
To be fair to this book, all of the Welsh material I've read (specifically devised for learners) have been fairly light hearted, which I can understand. When you're getting to grips with reading in a second language you might not want a heavy topic, but this is much in the vein of Caru Canu a Stori. Personally? I don't mind that, as long as it's marketed as such. What Cyw has on offer (including the legendary Byd Tad-cu) and Ceffylau have a place in my Welsh learning materials when I want something soft and simple in my second language, but I'd perhaps aim this at older children learning Welsh, rather than adults.
Still quite nice tales, though - just take with a pinch of salt.