First of all, with a stock OPLL (same is true with an OPL and OPL2), the final three channels can either work as three instrumental channels, or as five percussion channels (for a total of 9 or 11 channels, with the front six always being standard channels). Those final three channels are removed from the VRC7 leaving just the front six channels.
Percussion is provided by the NES' stock 2A03 white noise channel. There's nothing preventing the other stock PSG channels from working in conjunction with the VRC7, but this game doesn't use them, except for some tom fill/hits (white noise plus a triangle wave portamento fall). Also, chip artists who have composed for the VRC7 have incorporated 2A03 PSG into their music.
But the biggest difference between the VRC7 and a stock OPLL are the "canned" instruments.
Both the stock OPLL and the VRC7 only have one channel that is completely "user definable", by which I mean the operators can be set any way the composer wants. The other channels can only pull from a pool of 15 predefined "canned" instruments, where the operators can't be adjusted (kinda like picking, say, tone setting 45 in the keyboards you see at Best Buy or Walmart). You can pick the voice, but you can't configure the voice.
That's how the other channels work. And both versions of the chip work this exact same way. But here's the difference: the pool of available canned voices on the two chips are not the same. The stock OPLL's set have been replaced with a new set unique to the VRC7, and to my ears, based on our lone official example, it sounds like the VRC7 has a much better, more sophisticated, more advanced, and more pleasing pool of stock instruments than the standard OPLL. Now, of course, that's not to say all 15 are different, just that not all 15 are the same.
Therefore, even with fewer channels, I feel the VRC mod, on balance, is one for the better (much better), as I feel this soundtrack would not have sounded nearly as good on a stock chip.
You be the judge. This may be the best, most complex sounding soundtrack I've ever heard made up of only two-operator voices (and it's not even running on an OPL2). It doesn't sound very vanilla FM to me at all....and perhaps most surprising of all, it's just an 8-bit NES game (well, Famicom, but you get my point.)
The great, the mighty, the legendary...Lagrange Point!!!