fixed will only work without problems if the thing to implement is specced to tiniest details

otherwise you’re also estimating the discovery process which isn’t under full contractor’s control

And even good specs can be disastrous if they aren't lined up with the reality of the system, or if there's political / coordination factors involved.

yes the unknown unknowns 😛

yep

How do you guys judge competence?

if requirements are likely to change, fixed will fall apart