If one hasn’t played a full campaign, I would suggest Veteran. The downside to Hero is that the ODI marches along quite quickly and it could be game over before one grasps all of the critical goals/missions/research. Veteran is more forgiving with the ODI progression. (see ODI in Leviathan 1.0.56049 - 04/3/2020 Patch Notes). With Veteran, carefully spaced & planned saves can help one use the way-back machine without throwing the baby out with the bath water. Also the faster the ODI, the less time one has to explore all of the different side mission and story lines. So if your goal is to milk the most out of the game in one play through, I would vote Veteran.
There could be some issues due to RNG placed bases, faction havens, special missions that can also break the game, and may not be obvious until one has spent several hours playing. There is no going back other than a new game.
This is my take on having played through on the first 3 difficulties (Rookie, Veteran and Hero). I am sure there are others that will share a different opinion, hopefully those that have played at least Veteran to Legend.
Then again, you may just wish to table the game until it’s closer to the Steam release date. As there are numerous planned changes to the game that are being promised.