This is the real problem - there are deadlines and most projects don't even consider working in questionable network conditions. Developers aren't going to put in the extra effort when it doesn't contribute to the job they are asked to do.
I always at least _ask_ in the requirements gathering stage for a new mobile app: "how much effort do we want to dedicate to app performance/reliability under marginal network conditions?"
As it turns out, pretty much all mobile app owners are as apathetic about that as most mobile app developers. (On the other hand, once you've got a reputation for being able to handle those sorts of flaky network edge cases, you get more and more work for the sort of apps that benefit from them. The downside of that is it's never the flashy resume-building-apps that come to you for this.)
As for how hard it actually is: iOS has already been explained, the Android emulator has network latency and speed simulation right in the GUI, as do most web browsers. And there's alway the option to switch your phone to 2G only in the settings and/or go into the basement or elevator.