HoI4's Romanian ones were actually quite fun, you really only have to beat the Soviets to get both in one run - not easy, but far less tedious than ones where you have to fight multiple or all factions one by one (like the one for Austria-Hungary). EU4 definitely takes the longest time as there are lots of nations with unique achievements, 89 by my count, which are impossible to get in the same campaign - several of them requiring playing until almost the finishing date while others have a tight time limit.