An FPS without emphasis on combat is a bit, wasteful wouldn't you say? Ok it's not a 'combat game' but combat has still been a prominent aspect of the Fallout series with many machanics devoted to it's singular function (not including Tactics as that was pretty much bred for combat ) I would have thought that the FPS spin on Fallout combined with it's history of being quite an original series would bring something fresh to the table from both sides of the RPG/FPS fronts. I don't think VATS plays enough of a part, and it's function is a one-trick pony at best.
As for overall difficulty, I find it hard to make FO3 challenging. It has 5(?) difficulty settings. Normal is too easy, and very hard is too normal, from my experience at least. I've considered playing FO3 on very easy just to see how much easier it can actually get I find the originals provide somewhat of a challenge on normal settings. FO2 is definitely more difficult when you change the combat setting to rough, I've experienced countless misses at 95% hit ratio, and at least a few critical misses (at luck 9) compared to zero on normal. I would guess it also gives critters more HP, or your damage is stunted whilst theirs is amplified, but beyond simple value changes the mechanics themselves are addressed, it's a very noticable difference once you make the change. And beyond that you can make the non-combat aspects of the game more difficult because it has a separate slider catered to that aspect. I also think Tactics is challenging enough in it's own right, I'm very keen to play again on very hard in my next playthrough, and I expect I'll be pulling my hair out based on how hard I'm finding it already on hard setting, ironically