I can't imagine they're considered the weak point.

Not weak, per se, I just get why they wouldn't be to everyone's taste. A lot of people are going to want more action, and that doesn't pick up for a little while. A bunch of people want to see Frieren blasting demons, and that takes a little bit to ramp up. Intro isn't bad, but some people just won't have the attention span for it, I think.