Volo's Guide to Monsters adds new giant variants for the six above. These include a new hill giant (CR6), stone giant (CR10), cloud giant (CR11), frost giant (CR12), fire giant (CR14) and storm giant (CR16).
The toughest of them, the Storm Giant Quintessent, is a nightmare in its own lair. It only has AC 12 (!!!), but it has a legendary action that allows it to teleport anywhere within its own lair... and a pair of javelin attacks that never miss any target within 600 feet for average 38 damage per round. If a dragon attacks one of those, it's in trouble. The storm giant (which can fly) teleports to a spot in mid-air, 500 feet away from the dragon. It tags it twice for 38 damage. On the dragon's turn, it double-moves (say) 200 feet closer; still out of range of its spells or breath weapon. The storm giant then teleports to maintain the 500 foot range... rinse and repeat, until the dragon (any dragon; any size) is dead. And, because it's a legendary action, the storm giant calls it at the end of the dragon's turn. Even if the dragon somehow closes the range with (say) a dimension door, it will never get an attack off.
The other "new" giants from VGTM have similar interesting wrinkles. The frost giant has "troll regeneration", which means it can't easily be killed by its typical prey (white dragons). The cloud giant has a range of spells (including invisibility and suggestion) and can polymorph into any beast it has previously seen. In Storm King's Thunder, you also see stone giants with absolute top order spells (no spoilers, but some 8th level parties are going to be very surprised if they wander into the stone giant lair...).
...but, putting aside the new (tougher) giant variants, it's all about the numbers. Storm King's Thunder describes longships populated by 20+ frost giants... and all the giants have dozens of servitor creatures (many with flight or missile weapons). Any dragon that can get close enough to use its breath weapon is also certainly close enough to receive a dozen thrown boulders.
As for fighting characters... sure, most giants are one-trick ponies. They're beef-sticks. From my own experience, a typical control spell like hypnotic pattern can end a giant fight real quick. A few sessions ago, the party absolutely dismembered a trio of frost giants at only 7th level by doing the following: barbarian rages, druid hits him with a polymorph (Giant Ape), then wizard casts invisibility (using a 3rd level slot) on both himself and the druid. The giants couldn't find either of the two spellcasters to disrupt their concentration spells, so had to wade through 300 effective HP (giant ape has 150 HP, but 1/2 damage due to rage) while the barbarian tore them apart with his simian fists.
Of course, on the flip side, if the giants win initiative and focus fire on one of the spellcasters, they can kill him outright. I've seen the party wizard knocked out from max HP simply because two giants got within reach. If one of those hits had been a critical, it might have back-doored him (negative max HP). So, it's all a bit swingy.
If you're concerned about the challenge for the group you're playing with, mix it up a bit as shown in SKT or use one of the new variants from VGTM. Pair the giants with some servitor creatures with high Wis saves, or make one of the giants a caster with dispel magic, or add a bunch of hell hounds, or whatever.
