Well, this isn't a black and white question. It's a question of odds, and many variables that affect those odds. Are your chances of getting a cross contamination higher when using plastic over glass and stainless? Without a doubt, yes. Normally people blame this on microscopic scratches in the plastic. However, I've also read at least one wine study that actually documented that it took longer to sanitize microorganisms off of plastic versus glass and stainless steel in a controlled experiment (I will have to revisit the study to see why; I believe they referenced other studies for that information). Also, I am not sure if the study cleaned before sanitizing, so I have to re-read that.
That doesn't mean that you can't sanitize plastic fermenters that have been exposed to a mixed culture fermentation. I need to read more into this, but it looks like if you do long soaks of first a cleaning agent (PBW) and then a 5 minute soak in sanitizer (Starsan), you should kill everything unless your plastic has a lot of scratches. Knowing if your plastic has a lot of microscopic scratches is, well, hard without a microscope unless you can see the scratches, at which point those scratches are no longer "microscopic". I can't imagine that any plastic would be 100% free of this. The one exception might be unless it was brand new. That all said, I have fermented at least two clean beers in plastic that have once had my sour culture in them, and haven't tasted a contamination. There might have been a contamination if I were to send the beer into a lab, but since I never tasted anything wrong, I don't care. On one of the beers, it was heavily hopped, thus most likely killing any lactic acid bacteria. It was also a very short fermentation (under 2 weeks), and then moved to a keg where it was kept cold (a keg that has held many sour beers, in fact). The cold would have helped minimize the impact of the contamination if there was one in my beer.
When talking about cross contamination, there are two worlds: the homebrew world which tends to get it's information second hand from the commercial brewing world or anecdotally from experience or the experience of other homebrewers, and the commercial brewing world. In the homebrewing world, contaminations often go unnoticed, which might be likely in my case. God only knows what variable might play a role in one homebrewer having a contamination issue versus another homebrewer not having a contamination issue. For example, we don't give our clean beers time to sit around at room temperature and express whatever contamination might be in them. Sometimes our palates aren't trained enough to pick up on contaminations. On the other hand, when commercial beers get infected, that means product loss, money loss, beer sitting on shelves going bad, pissed off customers, etc. Therefore, commercial brewers take contamination much, much more seriously than homebrewers do, and rightfully so. There are textbooks written about it, $1000 courses available from UC Davis about it, and peoples' jobs dedicated to it in commercial breweries (this is called "Quality Control" or "Quality Analysis"). In the homebrewing world, we do our best to understand what is going on, but in the end it's just homebrew and no one is going to lose their job over it, if it even gets noticed at all.
That last paragraph was just a bit of streaming consciousness after having a nice cask strength whiskey, hopefully it has some sort of insight.
