Opeth for years on doom-metal.com have been argued if they are or even were doom metal. I say the first 3 albums definitely and can argue their sound was somewhat similar to Katatonia on their first two albums. There are traces of doom on many songs even on Watershed but not clearly enough to be considered. They are way too versatile for any one sub genre to be called. Plus it doesn't help the fact they are always changing their sound on each album and definitely after every 2nd album.
Katatonia has been argued back and forth what their newer albums are as well and the progressive term came in many years ago. I don't have a problem calling Opeth that either but I'm tired of the labeling. They are still metal so I just prefer to call them that.