The thing with some SOTM effects is that people ignore the fact that in most cases the damage increase is equal to having the ultimate at next level, which is the difference between a level 6 and a level 11 ultimate, which is the case with witch and pyro I think but you don't really pick them up on them because it's single target.
Hag doesn't get a "whole level" of damage improvement (it's a bit less), but the fact that it's AOE more than makes up for that, it also reduces the cooldown by 55 seconds at level 16, that's a big CD reduction considering the staff effect is basically free with the cost of the stats you're getting.
At the end of the day it's all down to your play style, if you're the only one doing anything, or you have specific heroes you need to lock down, hellflower is great because you can pretty much kill anyone with it, same with sheepstick, competitive hon players could tell me not to get a staff and I would still get it, it gives me what I want and I can win games with it.
Nah, hard build up and expensive + you dont have the mana early.
You have the mana after HF, and you don't have the attack speed + damage to build a damage modifier before HF anyway. The build up isn't that hard, only the 2700G item. And in any case, early on you're doing all your damage with skills anyway.