I have no problem with varying bounties. It is a problem when all of the bounties have a elevated threat level because I level up faster than I can upgrade my fleet. When I have one or two destroyers and twice as many frigates, and all bounties have two or three cruisers and more destroyers and frigates than my entire fleet, and better quality than my ships on top of that, that is a problem.
It sucks when all of the bounties that will pay are very easy and the optimal strategy is to shelve half the fleet.
It hurts more when I need to shelve my whole fleet except a Dram and my starter Wolf (for contact missions) because all of the bounties are too powerful.