It will get everything killed. The one time it works out for you will be totally sweet though.
AoE spells are just fun - Colour Spray, Grease, Web, etc - but will wreck your fighters unless you place them carefully (and the AI is pretty suicidal). Eventually, there are a lot of low-threat opponents, which means even the low-level ones stay useful the entire game.
Usually the most dangerous threat is anything that can throw bullshit magic back at you - so, monsters - and then worse, anything that can do that magic and has protections against your magic - mostly wizards. Most of those fancy monsters want you to stock up on a couple of status-healing items when you notice them for sale, just in case. Wizard fights become about comprehensively dispelling their protections and then them instantly exploding from Minsc slapping them with a greatsword.
Later, in BG2, things get pretty damn unfair (in both directions - a lot of near-impossible fights are totally negated because you scrounged up money in the starting city to buy an overpriced shield).
Irenicus’ backstory and the overall plot post-Spellhold are random and silly.
Irenicus’ voice acting and most of his lines are great.