The problem with the dungeon crawling in XII was threefold.

First, I didn't like the combat, and the mindless trial and error of the dungeons did nothing to shorten the amount of it I had to experience.

Second, the dungeon design was no good. I love good dungeon exploration. Games like Golden Sun and Legend of Zelda, with their puzzle and exploration based dungeons, are among my top games ever, and I would love to see some of that sort of stuff in an FF game. But this game didn't have it. The dungeons were rarely anything more than "find door, look for switch to door, press switch to open door, rinse and repeat". But it gets worse. Take the Great Crystal (which is by far the worst offender in the point I've reached in the game so far). No map, and not even any distinguishing features from one room to the next. If you didn't do trial and error for hours drawing your own map (or steal one off the internet, or, as the developers hoped, buy a strategy guide), you had pretty much no way through that place. That is not good dungeon design (and hasn't been since the dungeon-crawler genre died so long ago).

Third, and perhaps the worst of all, there was no point to the dungeon exploration. Say what you like about the boring linearity of a place like the Mi'ihen Highroad, I still feel it was better than 90% of the areas in FFXII, because it was interesting. There were a ton of NPCs along it to talk to who would break up the simple jog down the path. There were several plot-heavy points that gave you background information. Maechen, Belgimine, Rin, Shelinda, Cally, Captain Lucille, Elma, Clasko, Gatta, Luzzu, Dona, Barthello, and, of course, the dozens of crusaders rushing around handing you supplies and building up the mood for Operation Mi'ihen. You knew why you were going down the path, and you still felt a connection to the rest of Spira as you did it.

In FFXII, you were so disconnected from the plot and characters that you never really knew why you were going through a dungeon, what your objective was, or why you cared. The plot and the dungeon crawling was so divorced it was like two entirely different games. You went from a couple minutes of cutscenes and city exploration to a couple hours exploring dull, repetitive, and uninspired dungeons.

I would take the interesting linearity of X versus the boring non-linearity of XII any day.