Complex is not the word I’d use to describe it. There’s more to do in the Splitter sure, but this is a case of looks can be deceiving. You’re essentially doing the same thing several times over, and in fairly easy ways to setup, I might add. More targets doesn’t necessarily scale with difficulty.
Freedom Fighters and Three Headed Serpent are complex because reaching all the targets requires liberal disguise usage and lots of moving about, all while avoiding a lot of armed guards and sicarios. The Splitter by comparison has two useful disguises that can be swapped out for at any given time, has many scripted kills that are not behind barriers, the guard population is surprisingly low, and in general feels pretty relaxed as difficulty goes. Heck you can waltz around the first floor of the facility in your starting outfit pretty much unimpeded.
And starting a shootout is surprisingly tricky given how, as described above, how low the NPC population is in the Splitter. Not saying it isn’t possible to screw up, but it’s not nearly as bad as other levels. In fact, all the clones are compartmentalized to their locations, and some of them don’t have guard overseers (as the clones themselves have guard AI) so any kind of retaliation is a lot less likely than other levels – including the base level and even several ET’s like The Ex-dictator and The Revolutionary.
The Splitter is not complex, certainly not nearly as much as the Fixer, which to repeat what I said, does not tell you about things you actually need to know to complete it. Relying on the unknown is what makes it both tricky and complex-looking, as you need to discover information that you aren’t sure is correct, and that’s partly what makes it frustrating. Even when you fail, the game keeps up the illusion of secrecy. It tells you how you failed (killed courier, handoff marketeer etc.), but those are not adequetely defined ahead of time. It is bad game design if you expect the player to figure out mission critical gameplay elements for themselves, which could’ve been explained in the breifing. In fact, IO had to spell out what to do on their blogpost when the Fixer re-ran in H2, so it’s clearly a fundamental design issue with the Fixer that players aren’t gelling well with.
The Fixer sucks. Always has, and always will do unless IO changes it to be more intuitive for the player. It is still the worst, most difficult, and one of the more complex levels in WOA, to this day. Having more clones to kill and therefore more opportunities to mess up does not make a level complex or more difficult; it just depends on the levels’ execution when playing it.