I think it depends what you compare BG3 to. It is much harder than BG1 and BG2, but that is because enemies in the old games are, to put it bluntly, acting stupid most of the time. Their default action is to attack whichever characters from your party they saw first. On the other hand you have some encounters which can result in a party wipe, if you don't know what abilities & spells the enemies use, such as the infamous mage assassin in the Friendly Arm Inn in BG1, or Kangaxx in BG2.
But I've been playing BG1 and BG2 with the Sword Coast Stratagems mod for years now, and that mod improves enemy actions. They attack whomever has for example the worst armor (which can be very dangerous when facing enemy rogues), and they will switch targets if they cannot do damage. So for me BG3 is just the right challenge: enemies are not acting stupid, but neither do they have instakill abilities like Kangaxx' imprisonment.
For example, I am currently running a party with Wyll and he has a summon imp ability. So I sometimes use that imp to lure enemies to the party, making them abandon their terrain advantage.
