I'm SS+ and I've had the Quorn chicken style and beef style chunks (new), and also the Quorn mince.
Quorn fillets, if you can get them, are not too bad - a small amount of onion, but I've rarely found more than one little chunk in an average Quorn fillet. Have also on occasion had plain Quorn sausages - not the sizzling, bramley apple or cumberlands though. There's also (from some supermarkets) plain Quorn burgers (not the mexican ones or quarter pounders, which have loads of onion and peppers etc added).
I like Quorn a lot, and have not noticed any issues re: weight loss or ketosis when I've had these, to be honest. I don't have them every day, which probably helps. That's my experience - but obviously it's your choice.