Sky Attack has 140 Base Power and 90% accuracy. Regardless,
Fire Blast and Hydro Pump have that trade-off that as far as I'm concerned was a huge deal when I first joined competitive. That lowered accuracy actually made people pickup Surf or Flamethrower instead because of the prior two missing too often. That's why those moves are balanced in comparison to the 100% accuracy SolarBeam. Sky Attack gets that extra 20 base power for a 10% accuracy decrease though, essentially balancing it further.
I don't think it would be broken, again, because of how little it actually affects. I can't imagine pokemon like Lilligant getting it, but maybe something that doesn't really have anything else going for it, you know? Kind of like how Prankster was given to really bad pokemon that basically serve just that purpose (Sableye / Whimsicott / Volbeat)