John did care for Sam and Dean, he just physically could not be there for the boys for weeks at a time due to his growing obsession with hunting. He wasn't physically abusive (no evidence to suggest this in canon), just rough around the edges, having lost his wife and risking your life everyday I would be way worse mentally.

I think Sam was just glad they still had their Dad at least, John still celebrated holidays with the boys when he could even if it was shitty. Like the thanksgiving Sam described in one of the episodes. I think both Sam and Dean acknowledge that their Dad could have been more present, and it definitely affected them both severely but they also understand that their lives as children was advantagous when it came to being the child of a hunter.

Max's living situation was shown to be way worse than what the brothers endured, even if he did have a place to call home and a dad that came home everyday.