I would nitpick the title a bit. Revenue is never easy. I think they should rephrase as "Revenue is relatively easier than profit especially if you have PMF". Once you hit PMF (which is where most startups fail), you can just pour money into growth and that's when revenue generation becomes relatively easier.
Usually bad unit economics. The whole "make losses now to jack up prices later" thing does not work in reality. Usually the whole market just gets poisoned and the only way out is to leave richer investors holding the bag
