The Totem battles were mostly very well done. Hilariously, the Yungoos one was much easier compared to the Team Skull grunt battle inside the trial area, with that Grudge Shedinja paired with the boosting Whirlipede. The Yungoos and Wishiwashi battles could probably stand to be a bit harder somehow.
Most of the single battles were still susceptible to the same problem of the Pokemon AI as a whole, i.e. the lack of switching against very unfavourable matchups making it very easy to setup boosts and sweep their entire team. This is no fault of the mod, but a few circumstances made it somewhat easier. The first is the very early availability of the TMs for Reflect and Light Screen, which can be purchased as soon as you reach the 2nd island. This is in contrast to XY, where Reflect and Light Screen TMs were available only postgame, and ORAS, where they were available in Lilycove City, which is a bit over halfway into the game. Reflect and Light Screen makes it easy for the player to set up on the opposing Pokemon by drastically cutting the amount of damage received, so being available early means that the player can start to build their team around setup earlier into the game.
Of course, in order to actually abuse Reflect/Light Screen and setup, you need Pokemon which can do the job well. And here is another aspect of NS/UM that made it easier: Rotom.
Rotom can be caught at Route 1, which is basically the start of the game and immediately upgraded to one of its appliance formes in the same area it is caught in. Even if you do not intend to cheese single battles with setup moves, Rotom is a very powerful and versatile Pokemon to be available so early into the game, having stats comparable to a fully-evolved starter, extremely strong STAB moves and good defensive typing in Electric + Levitate and whatever type its particular current forme has. It is versatile in that you can easily swap its typing and STAB move anytime by going to Kukui's Lab. But more importantly, it has very good defensive stats and can learn both Reflect and Light Screen. Later on, it can learn Volt Switch (Mt. Hokulani), which can be paired with minimum speed to allow your setup Pokemon to switch-in at full health, and Will-o-Wisp (Konikoni City), which is amazing for neutering non-fire, non-Guts physical attackers. Its STAB moves are also perfect for abusing Z-Crystals as they have high base power but many drawbacks that Z-Crystals nullify.
Personally, while I think Rotom is a bit broken to be available at the start of the game, I really enjoyed using it since it was a very strong glue to the team, having crazy high stats at the start of the game and later on, Will-o-Wisp support along with Levitate to cripple all the Earthquake users that would otherwise hurt a lot of setup sweepers. It should probably be moved elsewhere, Thrifty Megamart would be thematic but pretty late into the game and Pokemon that are found late into the game are often not very useful.
And that leads me into the biggest gripe I had with the Expanded build, though it's not entirely the fault of the mod itself. That is, the move relearner in SM has been moved all the way to the Pokemon League, which is basically the end of the game. This was probably done because the move relearner was changed to be capable of teaching a Pokemon any move it can learn via level-up, including moves it would learn at higher levels. The problem is that because of the higher level curve, you will encounter a lot of Pokemon at very high levels, which locks them out of a big part of their learnset. This was not an issue in Rutile Ruby/Star Sapphire because the move relearner was in Fallarbor Town, which was pretty early into the Hoenn, but it's pretty annoying in Nova Sun/Umbral Moon when your teambuilding choices are rather limited because wild Pokemon can't learn a lot of moves they should be able to until you reach the Elite Four.
The team I used to beat the Elite Four ended up being Rotom-W/Porygon-Z/Tyrantrum/Gyarados/Flygon/Pyukumuku. The strategy used was to simply set-up screens and maybe burn the opponent, then switch to the appropriate sweeper, setup and oneshot everything. All of my setup sweepers had some way to boost Speed to solve the problem of trainer Pokemon now having speed EVs, with Porygon-Z using Z-Conversion while the rest used Dragon Dance. Tyrantrum was chosen because it learned Stealth Rock, which meant it could sweep without being stopped by Sturdy and Focus Sash. Gyrados was the setup sweeper against special attackers because Tyrantrum had terrible SpD and would die to special attacks even with Light Screen. Flygon was to setup on Electric-types, which caused problems for Gyrados. Porygon-Z was the backup sweeper for all other cases, though it usually ended up being the most consistent setup sweeper. Pyukumuku was there solely to check opposing setup sweepers with Unaware + Toxic/Recover/Soak/Protect stalling but I didn't use it in the Pokemon League because none of the Elite 4 used setup moves with their first Pokemon. The most annoying thing was the Skarmory with Whirlwind that I had to kill with Rotom first, but otherwise the rest of the Elite 4 was swept rather easily.
