When the advertised speed is "up to XX Mbps" and you occasionally do get those speeds, then what is the proper complaint? They are providing "up to" the speed purchased. It works very well in the provider's favor.
FWIW, even if you don't complain, it will help Netflix gather data that they can use to complain on your behalf.
There is no way to complain, but if we had some open, trusted services similar to speedtest.net to publish the effective speed it would allow people to make informed choices when selecting an ISP.