doesn't the Time Turner ruin everything.

The whole point was that the more time you went back, the worse it could be. A timeturner had to be used wisely, diligently, and in a limited way only. Otherwise, the consequences could be enormous.

An hour or two would be relatively less-dangerous, but it still could only be trusted to the most fastidious and trustworthy wizards. In the end, that would be very few wizards at all, as the temptation would be great to use it for nefarious means.

sorry George you can't have your brother Fred back, but Cedric Diggory let's bring him back and ruin his life...

I always thought that was a cheap way out. "Oh, uh...all the time travel is, uh...gone..."

The real problem, I think, is that the device was never put to good use before they were destroyed. So much pain and suffering could be avoided through the use of the time turners, but they use them to...let Hermione take extra classes. Oh, and save Buckbeak. They alter time to save the hippogriff, but not Harry's family, or Neville's, or anybody's.

Yep. If the Ministry controlled the Time Turners, when Voldermort takes over the Ministry he now has control over time. That's not a positive outcome, all history would be his to do as he pleased. If the Order of the Phoenix or any other good witch/wizard had them as well, then there'd be a time war.

Those are very hard to win, as any person on any side can go back and alter the outcome of any event or battle. The Death Eaters have no qualms about doing heinous things to win, as they stand to gain all of time as their dominion. Meanwhile the good, positive side has morals which slow their effectiveness against ruthless enemies. Too many factors at play for a win for the goodies without the use of Deus ex machina. Basically the Deathly Hallows and the horcruxes are those already, divine interventions against an enemy that will stop at nothing to win power.

Time battles and consequences are also very complicated to write, I'd assume, pointlessly convoluted mess story wise, as the Cursed Child debacle demonstrates rather well.