I think that BG2 is harder, and should be. You may have access to stronger abilities, but so do your opponents. However, there are 2 factors that I see making BG2 harder than BG1:
1. You are limited to only 5 summons. One of the sure-fire tactics to handle any hard encounter in BG1 was to send a horde of creatures at the opponent; generally, you would keep the wizards pounding out summons while your party's archers picked apart the opponent. In the final fight with Sarevok, my computer was running at a snail's pace, due to all of the summons I had out! By limiting you to 5 summons, this forces your party to actually engage many of the harder opponents (i.e. forcing you to actually be heroic, instead of hiding behind your summons).
2. Most of the protective magics that make creatures harder in BG2 did not exist in BG1. Especially during the later parts of the game, you are spending more time knocking down opponent defenses than you are actually sending attack spells at the opponent. In BG1, you usually could just send your fighters in at a mage, hacking away at stuff like Mirror Image and Stoneskin until they fell. A couple of magic missiles from your wizards, and the opposing mage was toast. You can't do that in BG2; try that with a lich and see how quickly he Meteor Swarms you to death. You may also have access to these abilities for your character, but overall a duel between magic-users in BG2 involves more "mystical jousting" to tear down defenses, while in BG1 it was more "disrupt the other wizard before he casts."
Just my opinion, of course!
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Death is only the beginning...