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In Harry Potter, we can see many wizards in many really important situations/fights using very ineffective spells, and so later on they pay the price for that (e.g. somebody uses expelliarmus, but later on he loses to somebody else or even gets killed by the same person he could have killed first).

For example, in the Harry Potter and the Order of the Phoenix, Harry used expelliarmus on Lucius Malfoy, even though he had two magic wands at the time (he lost the second as well few seconds later, but it doesn't matter now), so he was able to keep fighting even though he could have been putout of the fight, using e.g. stupefy. Then, after Sirius gets killed by the Bellatrix, Harry goes after her and uses crucio - one of the three Unforgivable Curses, same as the Avada Kedavra, and he could have gotten rid of one of the most important of Voldemorts servants.

Is it due to it being hard to cast such a powerful spell, and they are afraid they would fail the spell in such a serious situation, or don't they want to hurt somebody?

From the Order of the Phoenix:

Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, 'Crucio!'

Bellatrix screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had – she was already back on her feet, breathless, no longer laughing. Harry dodged behind the golden fountain again. Her counter-spell hit the head of the handsome wizard, which was blown off and landed twenty feet away, gouging long scratches into the wooden floor.

‘Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it - righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson –’

There is something beyond just casting these spells (I've never heard of Expelliarmus not working because the caster wasn't really sure they wanted to disarm someone or Alohomora not working because you weren't sure you want to find whats behind the door). They aren't as effective if you don't mean it.

For Imperius curse:

“They’re Imperiused,’ he added, in response to Hermione and Ron’s confused queries about Travers and Bogrod, who were both now standing there looking blank. ‘I don’t think I did it strongly enough, I don’t know …’

And another memory darted through his mind, of the real Bellatrix Lestrange shrieking at him when he had first tried to use an Unforgivable Curse: ‘You need to mean them, Potter!”

We can see that perhaps some level of skill or desire is needed here - the memory of the earlier event seems to imply it is a desire or malice that is needed.

More specifically about Avada Kadavra:

“Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nose-bleed. But that doesn’t matter. I’m not here to teach you how to do it.”

And Snape uses it to kill Dumbledore - I doubt he really wanted him to die - so Avada Kadavra seems to take power, rather than desire. Voldermort is spoken of as one of the most powerful wizards and uses Avada Kadavra frequently throughout the first and second wizarding wars. So it seems power (and perhaps practice) are factors with this one.

In a battle perhaps they just don't have the freedom of a spell which may go wrong. Better to go with what they know and disarm the opponent than attempt a spell too powerful and be disarmed yourself.

This seems to be due to some sort of "morality" that some of the good-guy wizards believe in, most notably Harry and Dumbledore. We know Dumbledore particularly avoided killing, as Harry stated at the end of Deathly Hallows:

"Master of death, Harry, master of Death! Was I better, ultimately, than Voldemort?"

"Of course you were," said Harry. "Of course – how can you ask that? You never killed if you could avoid it!"

And Harry had a bit of a heated exchange with Lupin after the battle at the beginning of Deathly Hallows. Some of Harry's choice lines from there:

"I saw Stan Shunpike.... You know, the bloke who was the conductor on the Knight Bus? And I tried to Disarm him instead of – well, he doesn't know what he's doing, does he? He must be Imperiused!"

"We were hundreds of feet up! Stan's not himself, and if I Stunned him and he'd fallen, he'd have died the same as if I'd used Avada Kedavra! Expelliarmus saved me from Voldemort two years ago,"

"So you think I should have killed Stan Shunpike?" said Harry angrily.

"I won't blast people out of my way just because they're there," said Harry, "That's Voldemort's job."

Although in that instance Harry was specifically talking about Stan Shunpike, whom he believed to be innocent, it seems from Lupin's responses (as I argued here) that Harry was against severely attacking even actual Death Eaters.

For whatever reason, Harry, Dumbledore, and perhaps others seem to believe that it is wrong to kill Death Eaters — even if that gives the Death Eaters have future opportunities to kill others.

In truth, though, for all his experience and D.A. lessons Harry is still a novice. He doesn't actually have a repertoire of many damaging spells. Stupefy and Petrificus Totalus might be a step up from Expelliarmus, but under normal circumstances the Death Eater can be freed from those within moments. Harry has never demonstrated the ability to perform Avada Kedavra, and his Crucios are mild at best. The only other powerful curse that we really see him use is Sectumsempra, and even that would not necessarily incapacitate an enemy, and could be counteracted by a knowledgeable wizard. So even if Harry did not have these moral compunctions there is not all that much more that he could have done. Harry is simply overrated. To quote our favorite Potions Master from the end of Deathly Hallows (my emphasis):

"– mediocre, arrogant as his father, a determined rule-breaker, delighted to find himself famous, attention-seeking and impertinent –"