I thought The Boogens on Blu-ray looked absolutely fine, though the film itself left something to be desired. Olive is a great company, so it's too bad they're almost out of horror films to release. I wish they could take the entire catalog of Scream Factory.
As for most disappointing Blu, I agree with the post saying The Deadly Spawn. Great film, awful Blu. I quickly discarded it and was happy I hadn't gotten rid of my Synapse DVD.