is because he has the reload power.
In terms of the bhaalspawn we meet, he's probably in the top 10 or so, on par roughly with Sarevok, Gromnir and Yaga-Shura, stronger than Illasera, Sendai and Imoen, but should be much weaker than Abazigal and Balthazar.
Think about it. There is nothing extraordinary about the protagonist except that he can turn into the Slayer, which is weak and frail compared to a half-decent fighter. He's only human or dwarven or what the heck ever the player chooses, and doesn't even get any fancy god like equipment the way Sarevok does.
It makes me seriously question the plot of this game as I play through ToB again. Abazigal is a dragon, so his power is obvious; he's strong enough that he's only twenty years old, and yet has a twenty year old son. That is some serious power(more like bad writing but meh) right there, not to mention being a dragon.
Balthazar is able to resist Time Stop. The only other character who can do that is Demogorgan, (and maybe the final boss, I forget) who is the prince of frigging demons; that a mere mortal, admittedly a demi-god but still only mortal, can ignore the most powerful spell in BG2, shows to his potential.
Now the protagonist; his powers are relatively minor in comparison. He gets some teensy abilities which are nice, but nothing nearly as powerful as Sarevok's armor and shield. His avatar of Bhaal is pathetic. You're better off as a straight fighter with a sword in each hand.
All of the protagonist's strength comes from sources of magic(stat tomes, machine of lum the mad, deck of many things, etc) which are far more powerful than he/she is, equipment which he/she wears, and the spells and abilities he or she uses. Without any of this fancy stuff, he's realistically quite weak. He can't even hurt the Big 5 without a magical sword, and if I recall right Sarevok is immune to non-magical weapons too in BG1.
The only thing the protagonist gets is the small benefits from Hell, which he only gets because Irenicus ****s up royal.
They tease and taunt us in cutscenes, showing us casting spells which ignore magic resistance and don't get a saving throw, obliterating Liches and Vampires and Mind Flayers, turning into the Slayer and ripping through Sarevok's massive HP and AC in three hits, promising power, but giving us nothing beyond what levels would provide.