"custom chipset" has become a buzzword people like to toss out as part of some sort of explanation why the amiga died, that doesn't rely on Commodore being idiots.
PC graphics chips were "custom" chipsets just the same as the amiga's were.
It's perfectly true what dammy said... In 1992 and after there was no way in hell Commodore could have kept up with the PC world, mainly because of the entire board being a custom chipset. it was, sadly, even in 1992, a doomed concept(for a PC, gaming consoles to this day remain an entirely custom chipsets all along).
If they went the way of a 68000 based mobo + PC expansion cards, things would have probably turned out pretty differently(strictly performance-wise).