All items which are not defined by default or via config don't have any default values for Impact, armor negation, and number of hits per swing.

One example,

Also, are crossbows and bows intended to have impact and hit-per-swing values?

You must set up the config file or datapack to add attributes to the modded weapons.

And yes, the impact value works as same as melee weapons. Hit per swing on the crossbow is same as Piercing enchantment.

I see.
Is there a way other than setting up every item one by one? like the automatic value generation in the old versions?

Actually, I haven't made an automatic value generation system.

Wait, then how did different weapons and armor that aren't vanilla or defined in config had different values of stats, like all axes having armor negation? Armors having increased stats in correlation with their protection value?

Ahh, I got it. They were hardcoded in older version but I removed them because it was waste of memory.

I'll consider how to revive a function like that.

Okay, that makes sense now. thank you.