Hi, Dominique,
Like you I have heard conflicting reports. The albergue is actually a bit outside the town of Mieres, in an old school. I know people who have slept there without complaint and I have also heard a lot of complaints. Apparently the albergue only opens late in the day (not till 5 pm) unless you happen to connect with the hospitalero when he is in a good mood and feeling generous. But I don´t say any of this from first hand experience, just what I hear from others. I do know that the albergue was closed for a rather long period several years ago, and some renovations were done. So I don´t know if all the complaints come from pre-renovation or not.
I had planned to stay in Mieres twice, but both times I got there early in the day and decided that since Oviedo is so lovely, it was worth it to push through to Oviedo and spend some extra time there. I do think that if I were going to stay in Mieres, I´d stay in a downtown hostal, but if you spend the night before in Pola de Lena, you are going to get there very early in the day and won´t likely find too much to do.
Let us know what you decide and how it goes. Buen camino, Laurie