Buffy's problems were caused by the fact that she was torn out of heaven.
No one bothered to stop and see if Buffy was okay, even after they found out about heaven. They just expected her to deal with everything - Dawn, the mortgage, bills, food, finding a job, saving the world. They just expected her to be okay. And she wasn't okay.
If her friends had taken the slightest bit of interest in her, Buffy wouldn't have been half as depressed as she was.
Initially waking up inside a buried coffin, then having to fight the demon motorcycle gang would have been pretty traumatic as well, plus, in her semi-delirious condition, having a conversation with a dismembered version of herself certainly added to the shock. If all of that hadn't have happened, if she had been, in effect, "eased," rather than "torn" out of heaven, things probably would have been better. Not perfect, but better.
But this wasn't like Angel. Angel's physical body was sent to a hell dimension.
But her friends actually saw Buffy jump through what couldn't have been anything but a hell dimensoin portal. Even though her body didn't go through, it's possible that her soul could have, or at least, that's apparently what they assumed.
But she died in the process, she didn't go into that hell dimension. Her physical body was still there. Her friends thought that her soul was in a hell dimension, and there was no good reason to thinking it. Willow should have known better, she would have to do more research before using black magic ...and there was a way to find out where was Buffy's soul. Angel used the Axis of Pythia to know where was Cordelia.
Bear in mind that Willow, at that point, was a very powerful witch, but she had little or no experience controlling that power. That's the screw up with the resurrection, not the spell itself. At that point, she was depending on the Urn of Osiris as a catalyst to make the resurrection spell work. Later,when Tara is killed, she doesn't need any urn to activate the spell, her chant is enough, and, when Osiris refuses to grant her wish, she is actually powerful, once more without any props, to punish him. Then, after spending time with the Coven in England and a few few false starts at home, she's much more cautious about casting any spells.when Tara is killed, she doesn't need any urn to activate the spell, her chant is enough, and, when Osiris refuses to grant her wish, she is actually powerful, once more without any props, to punish him.