As someone who makes ntr game myself.
NTR is really the hardest genres of eroge to do right. Because it is not just porn, its the context that give its porn value. Aka the story/characters/plot. The moments like character expressions, social cues, side eyes, someone looking away, all of those make up good ntr game... In fact I would risk opinion that they are more important that action itself. It is the anticipation that "sells" it not the act itself. It's the unknown in front of you.
Saxy also picked up a landmine on their hands in form of cuckold. Netorare is much broader as it involves cheating as well which gives you avenue to play around. Cuckold by definition removes cheating aspect so story/characters/plot become even more important to drive characters forward for it to make some sense and not get stale quickly. If MC accepts cuckold then naturally it removes a lot of angst etc. But in VR case MC still has that angst in him... which is a huge plus and strong part of VR.
Back when I designed my own game it started as two couples in a house. And without some overarching story with some major events it was just dead boring after few meet ups.
NTR/Swinging/NTS etc. gravitate around that spice we crave but at the same time in order to get that spice it needs major events, changes... the threat that something can change that changes relationship.
NTR games with limited changes can work in short games. But then again, you can't attach as much in short games to characters.