Part 1 is basically just more of Doom eternal but pushed further and harder, it may have been just me but I think part 1 is harder than part 2.
The three levels are all unique, large and well constructed with the 2nd level in the blood swamps being my personal favourite.
There are afew new enemy types that shake up gameplay a bit and the spirit enemies are especially challenging to deal with in a large brawl and the final boss was decent, nothing amazing but it was interesting.
The visuals and landscapes are just as breathtaking as they were in the main game but overall part 1 suffers from just being very similar to the base game which is definitely not a huge problem if you love it and I do.