Hi Redwake
Unlike SH3 and SH4 with Steam SH5 is a little more complicated because the game needs TDW's Generic File Patcher to make the game playable.
Without this application SH5 is unplayable whether you add mods or play the stock game.
This is because unlike SH3 and SH4 which can be played with Steam without too many problems Ubisoft introduced a forced Uplay account, internet connection for game verification, Tonnage Bar, and Campaign Icons to the SH5 game.
If you buy SH5 to day and you get a broken game with the tonnage bar and Campaign Icons broken.
Adding mods to Steam is not a problem.
If you are new to SH5 look > > HERE << STEAM is not compatible with the essential application TDW’s Generic File Patcher
Steam Conversion Installation Instructions for SH5 Updated Version
Peter