The last 3 hours are better gameplay-wise because you actually can use some vigors that should have been introduced much earlier in the game.
Bioshock Infinite has a serious problem with how its combat arenas are designed: They are often a bunch of scattered platforms connected by the sky hook system, with some boxes or pillars littered around for cover. These platforms are far apart, but are usually pretty small with the exception of 1 or 2 large central platforms.
What this means is that most combat will take place at mid-to-long range unless you skyhook over, at which point you're right up next to enemies. Unfortunately, most of the weapons in Bioshock Infinite are designed for mid-to-close range, especially the most common weapons enemies drop. Since you can only carry 2 weapons at a time and ammo is scarce because there are 2 pointless variants of every weapon with their own ammo systems, this leads to players often lacking a long-range weapon. So they have to constantly use the skyhook to reach enemies. And once they get to the enemy on a distant platform, they will likely be in spitting range.... Which is where you can just beat foes' brains out with the Skyhook.
For most of the game, vigors basically do one thing: Shoot a projectile. The actual effect of the projectile will be something that deals extra damage or immobilizes the enemy. They act as better guns, which makes many of the actual guns feel even more worthless. The only one that really compliments the guns is Bucking Bronco, but it still doesn't really address the issue of distance.
Charge and Undertow massively improve the combat because they let you more easily control distance. Undertow lets you bring enemies to a range where your weapons are effective, while Charge lets you just zoom around to whatever enemy you want. Suddenly you aren't entirely at the mercy of the level design for which distance you fight enemies at. Of course, Charge is hilariously overpowered, so it kinda breaks the combat, but at least it's more fun that way.
Basically: Bioshock Infinite's 2 weapon limit and spacious arenas need late game vigors and the associated upgrades to actually work on a fundamental level. The enemies are still braindead bullet sponges, but at least you can make your own weapons more satisfying.