If you're living in the Western hemisphere, MechWarrior 5: Mercenaries technically launched on Steam, GoG, and Xbox last night, but everywhere else it launched today along with its $20 expansion, Heroes of the Inner Sphere. You can pick up the base game for $30, which has also received a massive free update with numerous improvements since MechWarrior 5 launched on the Epic Games Store over a year ago.
Now it doesn't matter what platform you purchase MechWarrior 5 on since the game has full crossplay with everyone. In fact, the built-in crossplay features allow Steam friends to play with Epic gamers or Xbox couch potatoes in any combination they like (up to a maximum of four since that's how many people you can have in a squad).
However, native friends list systems will only allow you to invite friends to play on the same platform, and Steam's native friends' list functionality isn't working at all. PGI plans to address the Steam native friends list issue in a future patch, but for now, make sure to use the in-game multiplayer features if you want to play co-op with friends across other platforms.
A lot of MechWarrior 5 players (myself included) enjoyed the game using a healthy array of mods. Unfortunately, the latest patch and DLC haven't played nicely with existing mods, and PGI warns, "It is advisable best to wait until your favorite Mod is marked as compatible with the latest update and the DLC before installing. We will be working closely with our mod-building community, providing them support and a direct line of communication with devs and each other in the Modding channel on the MechWarrior Discord server."
It might be a good idea to see what the base game is like without mods in the meantime. MechWarrior 5 received a free update that improves multiple facets of the game, from performance to enemy AI, while Heroes of the Inner Sphere adds new 'Mechs, new mission types, and new sidequests featuring famous MechWarriors from the wider BattleTech universe.
MechWarrior 5: Mercenaries and Heroes of the Inner Sphere are available now on Steam, GoG, Epic, and both Xbox One and Xbox Series X|S.