I came here just wishing to point out this: with patch #4 released on 11/2/2023,
BG3 has finally, correctly imposed the -4 damage penalty on every crossbow hit from off hands, unless an in-game feature/item is taken to remove the penalty.
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Unfortunately, I read the more recent posts...
BTW, I agree that the original post was indeed confusing, because it has unnecessarily included a lot of "irrelevant" information. Its most confusing part is its prominent emphasis on "jake of all trades," which can be an in-game class feat, a unique BG3 achievement, or just a regular phrase that merely means being equally good at lots of different things.
A better title for this thread may have been: "Archery versus Eldritch Blast?"
Or more to the point: "Dual Hand Crossbows vs Eldritch Blast"
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One more comment: Number crunching has its limitations and can be confusing and misleading.
I did a lot of number crunching when I was replacing "lightning charges" with "reverberation" for my first-playthrough character, which is a Dark Urge. I'm not bad at math. However, I know for a fact: past a certain point, such number crunching is going to be too simplistic to be still meaningful.
How so?
Just think about this: if a player character in BG3 can genuinely do an average of 51 or 48 damage per hit (as numbers may have misled us into believing), assuming the same character can naturally attack 3 times per turn, the average damage output per round of this character is about 150 per round. Given 10 full rounds, if not disrupted, this character can do an average of about 1500 damage. By such numbers, how long do you think it would take for this character to take down a sitting-duck monster (who can't move at all) with only 300+ HP in Tactician mode? Surely, it has 50% resistance to piercing damage, but that only makes it equivalent to a monster with ~700HP but no resistance to physical damage. To do a total of 700 damage should not need "1 grueling hour" or longer.
The total damage output of my entire Dark Urge party was only about 100~150 per round at the very best at the end of Act 2, not nearly as good as a solo character doing 150 averagely. But I do know for a fact: during my first playthrough, after my Dark Urge talked to the general, the entire ensuing battle was over in only 3.5 rounds. Yes, both the general (over 100HP) and the Apostle of Myrkul (300+ HP) were killed in a total of 3.5 rounds. No potions, no tricks, no pre-emptive strike. All 3 of my companions were stationed near the entrance when the battle started. I only followed what the in-game hints suggested. Everyone fought in a way fair and square. And that was my very first time being there. Only 1 of my companions took more than a little damage. It was a very easy battle to my Dark Urge party.
I think the lesson that could be learned is: The so-called number crunching in simplistic calculations does not paint a full picture.
Numbers can be very helpful, but they are meant to be suggestive only.
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The missing stuff, the truth, is:
In my opinion, while a full player party can fight every boss in the game fair and square yet never gets defeated even on Tactician, a whole-game solo char in Tactician mode has to find one or more ways to ultimately counter the underlying combat mechanism of this game. Yet, they can be defeated many times over (forcing many reloads thereafter). Which essentially means, the way how a solo char defeats the game is not something that simple number crunching can tell.
Here is another example that we all know: A level 2 Warlock can easily attack many bosses without facing immediate retaliation. However, number crunching cannot tell us why. BTW, I prefer to fight fair and square. So, my Wyll's (multiple versions) have never used such tricks in actual playthroughs.
Numbers have limitations. After all, BG3 is not a number game. I think the main reason why the original post failed to have solicitated an "acceptable" answer is because players in general play BG3 as a game, not as a number crunching home project of little merit.