Hi Richard. I think that the main thing with brett is that it's a matter of degree. I only do wild fermentations at ambient temperatures in my cider room where temperature is controlled through opening a window at night to cool the cider. A certain amount of barnyard, bandaid, and occasional buttery notes should be expected with the wild yeasts and low sulfite regime that I practice(50ppm total). What I've found is that it's the amount of bret that really matters. My wife has a very low tolerance, so she really doesn't like any amount of it. Other people that drink these ciders love them. I think that once it's the only thing that you taste, or at least it becomes the dominant flavor in the cider, I feel that it becomes a fault. This is obviously very very subjective, and people that drink natural wine and cider with little or no sulfites probably have a higher tolerance. I'm amazed that some of the commercial ciders I've tasted were ever released to the public, but apparently there is a market for these extremely funky ciders.
In my experiments at home, temperature and fermentation speed make a huge difference in the end results. Also, the use of sulfites will inhibit some of this. A small dose of sulfite(25ppm) at bottling has kept the brett in check on my more recent efforts. One "mistake" that I made last year was I over-racked some of my blends, and they were still fermenting extremely slowly in June, July, and August, when the cider room was warm. These all had a higher level of barny and bandaid notes. Not terrible, but definitely funkier than the ciders that were bottled pét nat with that small sulfite dose prior to the cider room warming up to 69F/21C. I like making different types of cider, so having varying results is a plus. This is obviously more difficult to live with at commercial scale. With true traditional "farm cider," a certain amount of brett should be expected and is true to style.