I think IGAU and I2 are more solid in their general style than MK9 and MKX, which I find have fatal gameplay flaws that keep them from being very good games.Both MK9 and MKX decided to hedge their defensive options around armor, and it failed in both games for different reasons.
MK9 barely worked, so of course trying to armor out of pressure often results in a terrible glitch and you lose something but gain nothing. In addition, character's fastest pokes were either full combo punishable on block or offered very, very meager hit advantage (Poor Scorpion's was negative!) for the risk of poking out of pressure. This made any pressure sequence, simple though it may be, nigh uncounterable with a lot of characters (Kabal vs. almost the whole cast, Johnny vs. a large majority of the cast, Sonya vs most of the cast, Shang vs. a lot of the cast, Jax vs. characters with high hitboxes, etc.). Unless you could push those characters away very easily (which a lot of the best characters could, considering they either had really good armor!) then it was hell trying to make it work. The risk/reward was horribly skewed, and the low tiers of that game feel like the worst characters you could ever play.
MKX at least works very well, but the problem is now that armor is exposed for the pretty weak defensive option that it is on knockdown. During pressure it worked fine because it did not have the flaws MK9 did, but on a knockdown most characters were essentially helpless. Between characters having the ability to run up for pressure, which lessened the effectiveness of generic tech rolling, and having only one timing in which an armor move could be used as a wakeup, a lot of top characters were free to run in and do quick strings that often broke armor before it could come out or they had a very long string that could cover the timings of both normal and delayed getup. A character like D'vorah, strong as she was, was almost helpless against a character like Kotal Kahn, who could do b122 on her knockdown and beat her backdash, armor, and delayed timing. This helped to homogenize the gameplay, as many others have pointed out, because as strong as armor was for fighting against things like traditional projectile zoning and defensive poking, it was just as bad for defense.
IGAU and I2 choosing to have invulnerability in addition to some armored moves makes for a greater dearth of defensive options, IMO, because it forces the attacker to consider it because no move they have can blow through it. Obviously in IGAU the risk reward of many of the wakeups were very skewed, but that was a balance issue that is largely removed in the sequel. Having these stronger defensive options opens up more avenues of play, which makes for a better game IMO, because variety, not balance, is the spice of life.