They work very well. Including the cheap ones. They do not have scalers or anything internal to manipulate the video or audio streams in any way. Most have a dip switch or dial to choose whether you want 5.1 audio for optical/coaxial output, or will have a stereo switch.
When you put these into 'STEREO', your source will see the sink device as a stereo only product. It will NOT transmit surround sound. These devices are not DSPs. They are not mixing from surround sound to stereo. They only accept PCM stereo audio to put out on the stereo connection on the unit. Stereo audio will still pass through the unit to the TV or whatever else is downstream on the HDMI, but stereo will be put out of the extractor as well.
I have switched to a system which performs a DSP downmix of surround sound to stereo, which is great in my setup, but not typical for most
extract music from video.
I have not seen any difference between the Crestron DA2 which is a top dollar HDMI splitter w/audio extractor and the units I've used from eBay.
I have never had a need to maintain surround sound and get stereo audio at the same time in any environment other than my home. So, my home gets the good stuff.