They're allegedly fairly effective (the claims are what, 90 or 95%?)... if both barbs hits. If it's not effective, you have no recourse. There are no effective failure drills.
Reloading simply takes too long, and as previously stated you only get one shot between reloads. If the very first shot doesn't work, you're in trouble. By the time you reload, an attacker can close the distance or get out of range.
Electricity running across someone's torso is not completely safe no matter what Taser, Inc. claims. It is less-lethal (very-probably-not lethal) rather than non-lethal.
The cartridges are expen$ive. How does anyone expect to practice with a taser?
Civilian tasers have 15' wires (not that 21' wires in LE versions are much better). If an attacker is much closer than 15', you have no hope of getting the taser out in time. The attacker has another option: if he sees that you have a taser, he can probably move out of range before you can use it. At that point, he can do whatever he wants. He can find some sort of shield and approach you again, or he can throw a knife at you, or he can take out a gun and shoot you.
If you taser someone as they're running away and they fall, that may break the wires, yank them out of the taser, or pull the probes out of the goblin's skin. At that point, you have to reload, close the distance somewhat to ensure a hit, and try again. No thanks.
They use batteries. Batteries die.
They use electronics. Electronics die.
Aim is in a way more critical than with a handgun because both barbs have to hit, and the separation (of the X26c - civilian x26) is 8 degrees (according to the website). At 15', that's 2.1' of separation (2.0927... and that's linear distance, NOT arc length). I don't know if the dispersion is horizontal or diagonal or vertical, but I'm not impressed. The area you can aim for and still hit with both probes is rather limited. It's important to realize that the full-body incapacitation illustration so prominently displayed on Taser's website does not take into account the difficulty of hitting a target with both probes, which at 8° of separation and at 10-15 feet is not trivial, and probably not much easier than a headshot with a decent handgun.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)