Do they underestimate/overestimate price effect of increase/decreases in underlying equity?
under because delta is betwee 0 and 1
Delta underestimates the price increase of an option and overestimates the loss. Delta assumes linearity, but the option price change as a function of the underlying is actually a curve which is represented by delta and gamma. It’s the same concept as duration and convexity. Delta is only an approximation.
^ don’t think that’s the question…
you are answering “does the delta over/underestimate changes in the call options price”
OP is asking “does the delta over/underesetimate the change of price in the underlying”
…although its possible OP meant to ask the question you are answering