Usually, it is used to characterize a development library or framework, as in,
Rails is an Opinionated Framework
So why is it that "opinionated" also seems to be used as a euphemism for hard-coding, discrete variables and subroutines, avoidance of data-driven process, or otherwise representing decisions as fixed structures in the code?
An old rule of thumb in engineering is, that the earlier in the fabrication that decisions are fixed, the deeper and more wide-spread is the impact. Regardless of the decision's impact as a cost saving or increase, making a decision early causes a cascade of ripples throughout the system. That's the trouble with being too opinionated: you have already made decisions prejudicially.
Researchers of old used to say that you should build one (or two) to throw away. Perhaps the tendency of programmers to be too opinionated is why. Perhaps it is also why so much of today's hot newest Web coding technologies appear to be legacy code before they even get released.