Some consider Shostakovich very uneven in the quality of his output. What pieces are considered of poor quality, and why?

Opprobrium is usually directed at the made-to-order Stalinist cantatas and film music. See for example Putting the Stalin in Shostakovich: pro-Soviet cantatas cause outrage. They are regarded as insincere, bombastic, and forced on him. People in the West know that they weren’t the kind of thing he took pride in, and prefer to ignore them.

I haven’t heard them, because, well, ditto. I’ll queue them up in YouTube today.