Party vs Party
At the time of writing I’m planning a mini-campaign of Madness at Gardmore Abbey, converting the 4th Edition adventure to 5th Edition. Which, so far, has been pretty darn easy. The encounters copy over fairly well, all things considered. One cause for concern was “the rivals” a group of adventurers who are also competing for the adventure MacGuffin. Thankfully, in 5e, you can choose to run classed characters as monsters, using simpler statblocks as needed. Which is great. I’ve done my fair share of Pathfinder, and the thought of running a rival band of adventures in that system gives me chills. It would be hard at low levels, let alone the 10th or so level expected from the adventure. Even with the simpler rules, juggling five different statblocks (on five different pages of three different books) won’t be particularly easy.
Yes, running a counter-party of NPCs can be pretty stressful at high levels, even for seasoned masters. I’m part of a rather large gaming group (13 or so), and we use to run different campaigns to fit people in without having parties that are too big for the DMs to manage. When such encounters happen, members of the other group are called as co-masters to use a NPC (or more), so the DM’s job is just to be a fair judge of the battle that ensues (and we had some pretty epic ones!).