I feel that to get accurate estimates you need the same people using the same tech stacks producing roughly the same solutions week in and week out. I imagine something like making slightly interactive marketing sites using Ruby on Rails every time, Active record every time, etc. These estimates will be perfect.

On the other hand, for a complex SaaS back end and front end doing something unique, a complex system to which you are being asked to add something groundbreaking, where you need to pay off various tech debt as you go in order to get the job done, where you are negotiating the requirements as you go along, and finding dead ends and impossible things that you can't do that you thought were simple and vice versa - I think it is hard to estimate. I think in that case estimation is a waste of time and it should be thought of like a "value train". If there is a genuine deadline (rocket launch) then best to manage it well with appropriate large buffers and dependency management. I don't see this being done anywhere, it is usually the business trying to haggle down the engineers on their estimates (even though the engineers are paid a salary so ???).