I have been running the Encounters since the Phantom Brigade and now I am starting Lost Crown of Neverwinter. I can honestly say that it has been a pretty good success since I can count on a regular group of players that come in as well as some new players that come and go. The only problem we have is enough room and another DM. The largest group has been 9.There are some sessions where I only got 5 players, which is pretty rare.
I know during my tenure as DM. I would add 1-2 more monsters depending on the encounter and party size. I also felt at times the encounters were on the easy side. So I decided to change it up.
The best example came at the conclusion of the Dark Legacy of Evard. The group was halfway through the final encounter when I had them roll perception checks. The group was successful in hearing a rogue band of Deathwatch soldiers approaching. The Deathwatch is originally in the Shadowfell group. I customized to a more appropriate level. The approach was they were a 3rd party sent from Gloomwrought due to what was going on and basically take anything interesting and leave no survivors.
The battle tide shifted from easily manageable to hanging on by the skin of their teeth. I was taking out pc's left and right that once barely got bloodied and I even outright killed a character. They did manage to defeat the rogue Deathwatch members. The cool thing was that everyone really enjoyed that last fight. It was really cool to have them appreciate the change up.