I find Firefox and Chrome's network-limiter feature to be "unrealistically unreliable" insofar as it limits data transfer bandwidth, and simulates some kinds of latency issues, but doesn't seem to implement things like DNS suddenly stopping working for 5 seconds, or sporadic network drop-outs, or how some things will arrive out-of-order, or an entire web-page blocked by a single synchronous <script> from one particular external host.

I agree. It needs to simulate a phone right on the edge of connectivity, with the random 5 second dropouts, followed by 50 Mbps but downstream only, followed by another dropout, followed by 1kbps up and down, etc.

That's the way real networks behave, and what real users have to manage with.