Hi KM,
As JoelD mentioned, split layering is off the topic of this thread but I thought I would reply because your comments about split layering certainly wouldn’t represent the experience of the large majority of the people who use them very successfully over long periods of time or the main reasons that the manufacturers use them either. I think that the few reviews you found (out of probably tens of thousands of people who use split layering) are probably confusing the feeling of the transition between the layers with feeling the split itself or have unusual issues or circumstances that puts their experience inside a very small percentile of the people who use them (see post #2 here)
Even over time … you won’t generally find that the sections are shifting and there are some advantages to having layers that can be replaced individually with a zip cover that for the large majority of people who use split layering would easily offset any ability to feel the different firmness levels on each side.
While one of the advantages of a component mattress is that you can do just that (replace the cover) … it’s also very unlikely to be necessary in a few years and the sections certainly wouldn’t be “moving around”.
Phoenix