I find my biggest hurdle with estimation is often one small requirement which turns out to require a large amount of work. Like, "have this element behave in [u

I find my biggest hurdle with estimation is often one small requirement which turns out to require a large amount of work. Like, "have this element behave in [unique way] on this page." Ok, well, that element was never even built to be touched individually, let alone have custom behavior. So now it's actually doubled the development time. This is something which can usually be caught during estimation, but occasionally it isn't something you're going to notice until you're actually digging into the code and implementing it. And if you're going that deep during estimation, you're already practically building the thing, so estimation is now taking up a significant amount of time and effort. I am not a super experienced dev, but I haven't really seen a way around these issues cropping up often enough to make estimation feel close to worthless. (that is, it's unreliable often enough that it's unwise to couple any timeline too closely to any estimate)