The UK has a similar concept in the well-regarded and successful Open University. They’re even accredited in the US.
I found the material of good quality, and the tutorials/lectures I attended were fantastic. I did, admittedly, feel that the level of support could've been greater, but for a predominantly distance learning education that's par for the course, IMO.
The debt is indeed a bummer! Still, their fees are lower than most other UK universities right now, and my study landed me my current DevOps position, so I'm very happy.
While I didn't personally feel like I needed support, there were others in my tutor groups that did, and it was apparent that the level of input from tutors varied wildly - I recall one that offered weekly 1-on-1s for anyone that wanted, and another that took days or even weeks to respond to any query whatsoever.
My wife is now studying at the OU, and her more contemporary opinion echos everything I said above.
I studied at a university that regularly ranks top three in the world, and felt exactly the same way...
I considered it for my studies, since my area's universities were pretty far away. They're as strict as the regular Spanish universities and you'll get the same degree.
Also, depending on the faculty, UNED books can be pretty bad (and expensive), although I heard this changed lately.
From what I gather from people in other countries, they also require you to study and to be competent, but it is extremely rare to find teachers and professors with ridiculous standards that you often find in Spain. So, If you have the opportunity, save yourself the pain. You'll still learn, be respected and it will be easier.