I think, for me at least, it's not necessarily wanting a darker ending so much as it is having a little bit of story whiplash. Ozymandias and Granite State are basically Walt brought low, his luck and schemes having run out, leaving behind a desperate man on the edge facing his end.
Feline winds up feeling strange because Walt is basically totally in control throughout. His luck is in full force with the car. He manages to get Gretchen and Elliot to dance to his tune (or at least appear to be doing so), and he sciences up an awesome and total defeat for his enemies. Jesse lives because Walt decrees it. He even schemes ricin into Lydia's Stevia, sealing off that little loophole.
So it's just weird, mentally, to go from Walt at the end of his rope to a Walt basically dictating the events of the finale. It's a solid finale, it just feels... Strange. It makes me wish for a five-minute epilogue where we find out all of his last schemes ultimately failed, and Walt died as deluded as he lived.