Anyone who says these writers "suck" has no idea what they are talking about. I know it's all opinion but the books speak for themselves.
Both guys had different strength and I think at their peak were at the top of their fields.
Johns had a great ability to make stories grand in scale that were also accessible to casual readers. I'm guessing his background in the entertainment industry helped with that.
His strength was taking characters with 80 years of continuity (sometimes incredibly complicated continuity), giving them a fresh start and jumping on point for new readers and keeping the most important aspects of the property's history. The JSA, Hawkman, The Flash (Wally years), Aquaman, and Green Lantern were obviously his greatest works. The fact that his stuff was accessible to new readers helped adapt it all for larger media. Most of the current DC movies are taking cues from his runs. You may not be a fan of him personally and they guy can't stick the landing of an event to save his life, but he will always be in conversations about who was the best JSA, Aquaman, GL, etc. writers of all time. That's quite an achievement. He's also one of the last DC writers to be make their mid tier characters sell well. His Aquaman and GL runs were always in the top 10 and out selling all of Marvel's most well know heroes.
Morrsion will always be my favorite comic writer and I think he is right up there with Gaiman, Miller, Ellis, etc. as the most groundbreaking and innovative creator of all time. Animal Man, Doom Patrol, JLA, The Invisibles, Arkham Asylum, We3 are some of the best comics ever made. And that's just his DC/Vertigo stuff. He's also arguably the best modern Superman writer for cripes sakes.
You think the work of Ron Marz and Devyn Grayson eclipse the guy who wrote All Star Superman, New X Men, and Multiversity?!? I know it's just personal opinion but daymun.