I'd say the reverse is true: a lot of bad games are rated too high.
I can see games falling in two broad categories: blatant trash VNs that idiots rate as 5 purely because of CGs, and games that actually try to be better games, or heck, to be games at all. Games from the second group attract a more discerning audience and as a result are rated a bit lower.
There is no way in hell actual games like, say, Karryn's Prison, Future Fragments, Last Sovereign, or SK Aura are on the same level as a DAZ linear clichéd incest VN. Not even fucking close.
So my impression is that complex games are rated more or less fairly (all the games I gave as examples are good, but not without flaws, so around 4.5 is a fair score), but there's a flood of grossly overrated VNs that makes 4.5 look poor by comparison.