An issue that comes up for me with both groups is encounter balance. With the Imperium group the main issue is the size of the party. I’ve got 8 players in that game, usually with at least 6 showing up each session.
4th Edition had a very straight forward encounter building formula, if you’re running a standard 5 player group. When I add in the extra player or players, the solutions either become large numbers of enemies or several very tough, mostly elite enemies. The downside to having to run a huge number of enemies is the time it takes between player turns. That’s a recipe for disaster as attention wanders as it is when it’s not a player’s turn. The alternative of having a few, very tough enemies is, especially if the encounter goal is to defeat all enemies becomes a game of attrition. The elites have huge numbers of hit points and even if your players are hitting, they’re going to blow through their daily powers. The 5 minute workday will suddenly appear. A friend of mine, Iserith on enworld.com is famous for designing encounters where the success of the encounter isn’t tied to killing all the monsters and in some cases it is even detrimental.
For 5th Edition I’m having an issue of making encounters challenging. With the exception of good rolling, I don’t feel like I’m really ever putting the characters in peril. I know combat is supposed to be quicker in this edition, but we’ve really never gone beyond 3 rounds of combat. I’ve tried following the guidance from the Dungeon Master Guide, I’ve tried using the take from slyflourish.com, and I’ve even gone with a a whole different system of calculations. . The last one almost works to what I feel is correct, but I still tweaked am encounter the last session to add one more enemy to make it feel tough.
In both cases, my players are very strategic and do a good job of taking advantage of terrain and features of the combat areas. It’s hard to fault them for that and I do try to have the npcs/monsters behave in a strategic manner as well. I guess this is just going to be more trial and error.