Nyzul: Supposing you play it very conservative and exit after each 5 floor block to save progress, you will very rarely lose (if ever). Just meeting one time a week for 4 runs per week, you're looking at 5 sessions to go from 1-100. Maybe one extra time to account for exceptional bad luck (d/c, freakishly bad floors), or fools who refuse to listen on lamps floors. Remember that Nyzul was balanced for level 75 players, and we have 20 extra levels now. It's a cakewalk.
The only circumstance where you shouldn't build fire path is of you have an empy or mythic.
That pretty much sums up my feelings too. It's honestly not as bad as it looks, and I'd totally advise just jumping in and doing it instead of wasting time worrying about it.
It's a worse path than some magian weapons, sure, but nothing actually difficult. Just a little time consuming. Even the most obnoxious trial (Wamoura) can be handled by slaughtering Wamouracampa in Abyssea-Attohwa for a couple firedays. If you're lucky, you'll be in a season where fire weather actually happens in places like Ifrit's Cauldron. If not (like when I did it), it's still not that bad as long as you put a little effort into being ready for fireday.