Actually, I compare it to X myself, and not just to excuse its faults. The games did have a lot of similarities, though each excelled in various parts.

There Sphere Grid was no less linear than the Crystarium was. You could only customize near endgame when enough Key Spheres became availible, and even then, the cost of backtracking meant that development was exceedingly predictable. Tidus, for example, almost always evolves straight into Yuna's path near the end, since that is where you hit the lock at the end of his. Auron's might be more useful, but you'd have to either save up Sphere Levels while waiting for the Key, or backtrack and waste a lot of Sphere Levels, neither of which is likely.

The Crystarium was a big improvement, in my opinion. The offshoots are plentiful, and short, and there is no cost for skipping them. The fact that you have several different jobs to work on that are all inter-related makes it even better. While it was hampered by the overall battle system and the terrible pacing, the Crystarium itself had a lot of potential and was a very worthwhile successor to the Sphere Grid.

X allows you to backtrack, yes, but in very limited sections until endgame. You can't backtrack from Kilika, from Luca, from Operation Mi'hen, from the Thunder Plains... Yeah, it all unlocks at endgame, but until then, it is just as forced and linear as XIII. If it wasn't for the towns and for the forced items/quests they put in that require going back, there would be no reason to backtrack in X (unless you miss a Treasure Chest, the way I did in XIII. Dangit! >.<). Now, I love the backtracking, but without NPCs to talk to or side quests to do, it's worthless, so the lack of backtracking itself is not the big problem here.

XIII's story isn't bad, even if it isn't as good as X's. But it also doesn't have Seymour ruining the whole thing. (SPOILER)Dysley pulled off the whole "Save the world through destruction thing way better than Seymour could ever hope for.

Tomb of the Unknown King was VIII. You are thinking of the Omega Ruins, I think.

The comparisons are made for a reason, and not just to excuse the faults of XIII. The similarities are painfully obvious at times. X did a lot of things better than XIII, but both games had a lot in common.

(SPOILER)The primarily linear path, the fact that your party is branded as outlaws/heretics by the power base, the fact that said power base is in reality misleading the people through fear and deception, Barthandelus/Seymour both state their goal as saving the world by destroying it, both parties are pushed to sacrifice a member to defeat their foe (become the Final Aeon, or become Ragnarok), the Crystarium/Sphere Grid system... The comparisons go on and on, if you have played both games, you can't help but notice them.