According to the people designing and building this stuff, PoS itself was a lot harder than merging it with the rest of the network.

For PoS there was all sorts of complicated game theory, a whole new protocol, and trying to figure out every possible way to attack it.

For the merge, it's basically just changing the fork choice rule of the existing network, so instead of going with the fork with the most work, it goes with the one being chosen by the PoS network.