I haven't found that true at all, but then I've never ran or knowingly smoked any 16+weekers. In the cupboard alone are several 8-9 weekers that range from cheese to pineapple to kushy to grape to pine to lime to champaigne etc, and all have different effects and strengths etc.
I've just finished a period running strains that go 10+ weeks, and TY mentions one reason I'm not partial to them. But the biggest reason, is I expected significantly better smoke if they got significantly more time - 2-4 weeks longer. That simply hasn't been the case. One - the berry Goji - is exceptional, certainly up their with the best from this quadrant. But the rest varied from awful [1], to very good, but no moreso than many strains that finished weeks sooner.
I'll run Gojis again, but anything else that requires more than 9 weeks I can't see in the future. I expect I can grab a bud of these Christmas sativas for a few Fat Cindys at some point.