I've not heard all of Queen's unreleased stuff.
Here is what I think of some of them:
A Dozen Red Roses for My Darling: I don't like it. Just synths and fake drums - 2/10
I Go Crazy - Should have been on The Works. Good riff, authentic drums, catchy vocals and a good feel. Without any doubt it's better than Tear it Up - 8/10
The Call - 2/10 - Don't know much about it. But I don't like it.
Robbery - 7/10 - Definite potential, If Queen had worked on it a bit more, it could have made it onto Innuendo.
Lost Opportunity - 6/10 - A bit patchy but if Queen had worked on it the song would surely be able for a replacement for Delilah.
My Secret Fantasy - 6.5/10 - Interesting mix of styles and experimentation. With more work on the vocals certainly could have replaced Delilah.
Self Made Man - 7/10- Would have fitted in well with the Innuendo album. Brian is a bit quiet though. It reminds me a bit of some of The Cross songs with the funky keyboard riff. I like the guitar bit in the middle with Freddie's tounge in cheek attitude
I Guess We're Falling Out - 7/10 - In my opinion would have been a nice replacement for My Baby Does Me or even Party. It has the right sound and mood. I think Freddie could have done a bit better with the lyrics.
Stealin - 6.5/10- Tounge in cheek, Fun. A style and mood of song Queen had not experimented with. Would have fitted in well with the Miracle.
Hijack My Heart - 7.5/10 - One of the strongest unreleased Queen songs. Good lyrics and Roger's singing is quite nice. Quite a good riff. Should have been on the Miracle.
Dog With a Bone - 5.5/10 - Okay and a bit rough. Quite a good solo though.
Soul Brother - 7/10 - Perfect replacement for Body Language.
A Human Body - 7.5/10 - Replacement for Don't Try Suicide?. Nice song.
I do wounder if these songs will appear as bonus tracks on the 2011 Digital Remasters.
I don't think you know what "unreleased" means.
Aside from that, you're probably right about the poor quality of some. Queen were never very good at B-Sides or extended mixes. With a few notable exceptions, like See What A Fool. Soul Brother, A Human Body, I Go Crazy and one or two others, they were mostly the crap that wasn't good enough to make it onto an album, or in the case of 12 inch mixes, the least possible effort that could be put into a song to make it different to the 7-inch version.