Much of the benefit of the K&N filter is the packaging. Aftermarket FI systems can't run an OEM filter box. I think a small performance benefit can be had with a well-maintained system, and there can be a degradation in performance if a filter isn't maintained. I noticed increased performance with an aFe intake on my Z4M, as have others, but it wasn't night and day. The main reason I would install a mod like that is to increase the volume of the S54's iconic intake growl, to make driving more enjoyable.
The MAF issues are not a myth, but are also rare and mostly due to user error. The MAF sensor uses a hot wire in the intake, and its resistance changes with temperature. The MAF sensor incorporates a separate temperature sensor too. The change in current, due to the airflow cooling the wire, is measured and mass flow can be calculated (using ambient temperature and change in temperature of the wire).
https://en.wikipedia.org/wiki/Mass_flow_sensor
If oil from the filter gets on the wire, it affects its readings. This causes the ECU to miscalculate the mass flow rate of the air, and therefore the amount of fuel required. This can result in anything from a small power loss to misfiring engine.
It's not hard to use the right amount of oil with the filter. I've used K&N for years on multiple vehicles, and I've never had a problem with the MAF sensor.