There certainly is nuance. There is nothing magical about marginal costs - if someone is pricing something at break even or 1% gross margin and never have any hope of running a profitable business at that price because of their fixed costs, it's not different from a competitive perspective - the price is unsustainable.
On top of that, there is nuance as to what goes into fixed v variable, how fixed fixed really is, how good your management accounting system is, how good you are at predicting things like product recalls, or insurance losses, or loan recoveries, or whatever other variables are part of your particular business.
On top of that, there is nuance as to what goes into fixed v variable, how fixed fixed really is, how good your management accounting system is, how good you are at predicting things like product recalls, or insurance losses, or loan recoveries, or whatever other variables are part of your particular business.