while therian has all of those moves, it does not have the sheer power and bulk of megazard X, and especially lacks the typing. Since ZardX has 78/111 physical bulk and it will be max investing in HP, it becomes almost impossible to revenge kille, and lant's 89/91 bulk isn't nearly as deadly. Just as notable is ZardX's access to reliable recovery, which furthermore allows it to shrug off all those attacks.
That's half true.
Ground/Flying isn't that bad of a typing offensively (if only Landorus had two STAB moves), and it's arguably better than Fire/Dragon defensively. It has 3 resistances, 2 immunities, 1 weakness and 1 double weakness. Fire/Dragon has 3 resistances, 2 double resistances and 3 weaknesses. Another thing is that Charizard takes 50% damage from SR before mega evolving, while Landorus takes 12.5%.
If you just compare Landorus's and MegaZard X's bulk, you won't notice Landorus's Intimidate. The main problem here is by far Intimidate, and the two immunities to a lesser extent. Landorus-Therian has way more switch-forcing power to let it set up. It's basically MegaMawile version 2. Let me show you an example (using Slaking here but any other Pokemon is valid):
-1 252+ Atk Choice Band Slaking Return vs. 252 HP / 0 Def Landorus-T: 234-276 (61.2 - 72.2%)
252+ Atk Choice Band Slaking Return vs. 252 HP / 0 Def Mega Charizard X: 294-346 (81.6 - 96.1%)
Without Intimidate, MegaZard X is indeed bulkier.
252+ Atk Choice Band Slaking Return vs. 252 HP / 0 Def Landorus-T: 349-412 (91.3 - 107.8%)
MegaZard X's bulk is also a little lower than Landorus's in the Sp. Att department.
252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 0 SpD Landorus-T: 356-420 (93.1 - 109.9%)
252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 0 SpD Mega Charizard X: 340-400 (94.4 - 111.1%)
Another important thing is that Landorus can hardly be revenge killed.
-1 252+ Atk Choice Band Mamoswine Ice Shard vs. 252 HP / 0 Def Landorus-T: 316-376 (82.7 - 98.4%) -- 75% chance to OHKO after Stealth Rock
This is the strongest revenge-killing move against Landorus in the game, and full HP Landorus doesn't die to it after Intimidate.
Charizard does have recovery while Landorus doesn't, but Landorus can hold an item while Charizard can't. Both have 3 move types that give perfect coverage, but if Zard has Roost, then Azumarill
(and Carbink) resist it's moves.
I guess it should be suspected at least.