I have lived in this house for as long as I can remember. Never have I ever thought to myself “hmm this floor feels like it could be flatter”. This is because the last person that installed the flooring shoved cardboard and drywall compound in areas where there was a bit of a divot. That combined with the underlayment and a thick floor board helped us not notice the inconsistencies. However, since the house has settled and building requirements in the 80s were more relaxed, this has lead to a subfloor with a myriad of inconsistencies.
Because the subfloor was less than solid, we had to lay a metal grid that was screwed down then set with thinset to reinforce it. After that came the dry pack concrete that was use to fill out the low spots and to further reinforce the subfloor. Finally on top of that, we covered everything with a thin set and that was made smooth so that we were ready for floor installation.
Although I never noticed a floor that was uneven, the floor always creaked. I can proudly say that after all these efforts, the floor is solid, flat, and quieter than ever.
If anyone is planning on replacing their floors, I highly recommend fixing the conditions of your subfloor before starting any flooring projects as that will greatly reduce the changes of squeaks or flooring failures.