I covered Shohei Ohtani's potential as a pitcher and a position player in a post last weekend, which you can read in full if you want more details, but the executive summary is that he's a potential top-of-the-rotation starter but not likely to be an average regular as a position player.
His swing is long and he struck out at a high rate in Japan's NPB, so if nothing else, there would likely be a developmental period for him here -- and he's not going to get the 500 to 600 at-bats a year he might require to improve at the plate and become average or better offensively. The wrinkle here is that he went to a Los Angeles Angels team that happens to have at least one lineup spot that Ohtani could fill and provide an immediate benefit, even if he's still a below-average hitter.