I ran this for two of my players and they each wanted to play one character so we went with gestalt rules and 25 point buy, so keep that in mind. Overall, it's a pure dungeon crawl without any real chances of talking your way through an encounter, but the players were aware of that going in so had the right expectations. My full group played through Mummy's Mask a couple of years ago so myself and the two players for Risen from the Sands were quite excited to revisit Osirion. It only took minor descriptive tweaks to have it as a follow-up with the module being affected by the events in Mummy's Mask, and the two PCs were people that were mostly on the sidelines during the events of the AP.
The descriptions of the rooms are really well done, it's a surprising amount of encounters for a Free RPG Day module that's only 10 pages of adventure, and the map is fantastic as it allows multiple ways to get to the goal. My players missed a room due to bad rolls, which made it so they didn't solve a puzzle but instead of brute-forcing their way through it they moved on and found another way through the problem.
Some of the fights allow creative players to use the terrain to their advantage, it's not all undead, and all of the encounters feel different from each other. The final fight is really hard. Thankfully there's something the players can find along the way that make it easier. Even with that they still a lot of uncertainty about the outcome.
The pregen characters are going to have a terrible time with the module so I'd highly suggest that you allow your players to make their own. As I said, I only had two PCs so went with 25 point buy and gestalt, but I'm confident that a group of 4 with 15 or 20 point buy would do about as well, possibly better, than my two players did.
This is by no means a guaranteed win for the group, but so long as the GM likes the module a group should have a lot of fun with it. We ended up laughing throughout it and having a great time, and the two players were excited that they might not survive the final encounter, especially when they both failed their initial saves.
