Many people would put MK as worst if they are rating on the total happiness/comfort feeling. Once you are in or on an attraction you're happy and feel fine, but getting through the crowds, waiting in line, navigating ECVs and strollers along narrow walkways, etc can quickly make you feel unhappy and stressed.
I know way back when (my 35 yo daughter was still stroller age), before they overbuilt things at Epcot, and after several hours in the crowded MK, it was a relief to be able to move freely around Epcot.
It's not only that. MK's food options are mediocre, and despite having "a lot of attractions" a lot of them are outright boring, IMO.
Plus, the truth is, MK is just the florida version of Disneyland that's slightly bigger in size (though it has less attractions), with a bigger castle. Nothing unique about it at all. At the end of the day, MK will always live in the shadow of Disneyland. It's just the way it is.
That said, I still like it a lot, and I completely understand why it's the #1 park of WDW, and probably always will be. It's definitely has the most to offer for families.