The extra vibration occurs only if you don't have proper fuel
distribution LOP

I have never seen an engine, GAMI equipped or otherwise, that was as
smooth LOP as it was ROP - and I've seen a lot of them. That's not to
say that I haven't seen a lot that were acceptably smooth - I have.
That's also not to say that I haven't seen engines that ran smoother
LOP with GAMI's than ROP without - I have. But those engines with
GAMI's ran even smoother ROP. When operating LOP, the fuel-air
distribution needs to be much closer to perfect than ROP for the same
level of vibration, simply because the power vs fuel curve is MUCH
steeper. By the same token, unless the distribution is absolutely
perfect (which it never is except maybe at one altitude and power
setting) ROP will always be smoother. I'm sure that at some point the
difference isn't important anymore (the vibration due to power
imbalance is swamped by other factors) but nobody can say with any
authority what that point is.

Of course all the piston airliners routinely ran LOP - but it's
important to remember that over the course of its life, the cost of
fuel the engine burns is significantly higher than the cost of the
overhaul - and thus LOP operation, which can easily save 10-15% for the
same power and speed, can be economically advantageous even if it does
measurably shorten engine life.

Michael