I can see the comparison to Mignola, although that didn't occur to me before you said it.
I don't know the reason Adams ceased doing work for DC in 1978. I always assumed he was just making far more with Continuity associates, his own company. I noticed Adams stopped doing work after his SUPERMAN VS MUHAMMAD ALI special, that seemed (for a while) to be his grand finale to comics. Years later in 1982, Adams finally produced MS MYSTIC # 1, and despite lucrative creator-owned contracts at that point, a second issue was many years in the making.
I said back in the Ernie Chan obituary topic that I thought Chan was the absolute worst cover artist DC ever had (circa Aug 1975-Oct 1976), at a time they had that awful pseudo-Marvel banner across the top of their covers as well. The one series I really liked Chan's art was CLAW (a pseudo-Conan book) and I'm hard pressed to name anything else I liked by him. Even on CONAN and SAVAGE SWORD I preferred other inkers like Alcala and Dezuniga.
And I don't understand why Chan was ever chosen as a cover-artist. At that time, they had Grell, Buckler, Ross Andru, Garcia-Lopez, Rogers, Aparo, and several other far superior artists to do the job.
