Delves vs m+ to gear for raid

It seems delves take 1/2 the time of m+ especially due to no group forming, you get guaranteed loot and can go to heroic gear fairly quick. I watched a very talented tank go 0 to hero in about 15 dungeons or so. That would be 30 delves. I know i did a mix on my main and got to 600 and then did raids with morning delve. Crests the set back for delves? Am i missing something?