People always complain about Illusion's repetitive design and lack of real improvement, but keep overlooking that's just how Japan's eroge industry is by nature. Japanese culture is conservative, nobody proudly tells their family they're involved in the porn industry, so majority of new programmers don't seek out a career in this industry; they only look to it as a stepping stone to get enough experience to get a job at a mainstream developer.
Another aspect about Japanese business culture is that their salary depends on how long they've worked at the company. There's a huge incentive for people to cement themselves into a career at a single company early, because every time they change employers, their salary progression resets to the bottom of the ladder. Age and experience doesn't matter, what's being expected and rewarded is loyalty to the company.
Those two cultural pressures results in eroge studios having a permanent shortage of veteran programmers and why so few try to make anything more complicated than VNs; companies like Illusion at least try, even if their efforts ultimately wind up feeling amateurish because their teams are literally mostly newbies.
People really need to lower their expectations for Illusion's games.