LWR Normal is easier than Long War 1.0 Normal.
LWR Classic is similar to Long War 1.0 Classic.
LWR Brutal is around as hard as Long War 1.0 Impossible.
LWR Impossible is really tough, unforgiving, and will sometimes feel literally Impossible.
Playing Ironman is like jumping all the difficulties up another notch.
It's probably best to start out a little easier if you've never played LWR before.
Larger UFOs and swarming abductions are quite challenging -- treat them with respect and do not rule out retreating.
Expect to have satellites shot down, to not be able to stop a lot of UFOs, to lose soldiers, to lose missions, and to lose around 1 country per month (give or take).
If you are wondering how your campaign is faring (especially after a few squad wipes), use the grade the council gives you at the end of the month as a good indicator. This grade reflects how well you are doing in your campaign and how likely you are to pass it. C or higher is a good grade.
Last, if you feel like being masochistic and decide to play on impossible: please expect to lose many (literally many) times before you even come close to winning. Expect the game to feel unfair and punishing. I'd recommend treating impossible as more of a "How far can you get?" rather then a "Can you beat the game?". Impossible in Long War Rebalance is much harder than that difficulty was in Long War 1.0.
Regarding the First Mission
The first mission is very challenging. It is intended to act as a litmus test to guide you in selecting a difficulty. If you fail the first mission you may consider dropping the difficulty a notch. If you pass the first mission with ease you may consider upping the difficulty a notch. Injuries and even deaths on the first mission are not unreasonable as it is intended to mimic the difficulty you would experience in some of the toughest battles in the mid-late game.
In Long War Rebalance, difficulty levels scale many different aspects of the game. Unlike Long War, UFO HP and alien stat bonuses from leader levels and research are scaled down on lower difficulties. Difficulty level also affects perks like the effectiveness of the Outsiders' Growth ability, as well as the chance for aliens to use Overwatch when dropping into missions or after using Launch.
Note: Outsider growth is disabled when using Second Wave option: (#26) Defective Outsiders
|First Mission Alien Count|
|Ease Into It (2nd Wave)||5||-|
Long War Rebalance is designed for players of all skill levels, offering a greater range of difficulties and even more Second Wave options to customize your experience.
In general, Normal difficulty is easier and Impossible is harder (vs Long War 1.0). Veteran Long War players who are new to this mod should seriously consider dropping their difficulty a level or two as the mod can take some experience to get used to.
The difficulty curve is also adjusted so that the game starts off easier and gets significantly harder as the game progresses.
While the curve looks smooth, it is only an average and an approximation. The actual difficulty of the game varies a lot. Two missions in the same campaign can feel like completely different difficulties.