I think a major part of the frustration is more the _assumptions_ around the work complexity. Like decision-makers more easily make invalid assumptions regarding the complexity of the software portion. A good PM will listen to the SWEs when forming these assumptions and good SWEs must be able to communicate about them.
This could be bias talking, though. Is it common for sales or support teams to be given milestones that are off by 50%?
Treat them as resources for remembering/exploring code libraries and documentation. For example, I needed to import some JSON files as structs into Unreal Engine. Gemini helped me to quickly identify the classes UE has for working with JSON.
This could be bias talking, though. Is it common for sales or support teams to be given milestones that are off by 50%?