That might have been a good job for sub-arc? 516 gr. 70 is common boiler plate. Don't hear too much of 516 gr. 50. There are some other good flux-core wires like Tri-Mark and a few others. Some users say Hobart has less spatter than Lincoln. That said, I agree on the worm tracking, slag inclusions and porosity with flux-core. I've done a bunch of flux-core and at times it would weld perfect and then have all kinds of problems for no apparent reason and of course in the hardest places to grind it out!
Be glad you could use flux-core. I've welded thick vessels that had to be done with 1/4" 7018 and preheat. Biggest weld I ever worked on was a 2 1/2" thick repad on a 2 1/2" thick nozzle out the side of a vessel. 100% penetration so in essence 5" thick of weld. What really added to the job was the repad was cut for a 24" nozzle but the nozzle was only 20". Of course the shell was beveled to get full penetration after gouging into the back weld(inside) and once it was filled up had to be ground flat for the repad. Then the repad was beveled to have room to get full penetration and once filled up was ground flat with only a 3/8" fillet weld showing. The outside of the repad required a 3/4" or 1" fillet weld as well if I remember right. It took 2 shifts of 2 welders close to 20 hours to weld that up. It took a long time for it to cool off too.