Battletech without DLCs, ie "vanilla", has around 90% of the overall content you are going to get WITH dlcs. So instead of going straight away for the Mercenary bundle, get the vanilla game, and if you enjoy it, get the season pass

They have some flashpoints for your career mode that are rather interesting, but their difficulty is not well balanced. Some of them require you to use smaller mechs against hard foes. By the time you get qualified pilots and some hope to win, you don't need the rewards they give you. Still, these flashpoints are worthy for their story (if you are a fan of Battletech lore)

The ECM mechs are not as useful as they seem. One of the first skills of a pilot disables them entirely, and also you cannot fight and stay hidden reliably at the same time. They have their uses, but not essential to me. Also, I could not gather enough parts to build one in my career mode.

More maps is always a good thing. No complains here.

So, to sum up. Get the season pass, or separated dlcs, if you want to experience the lore of the flashpoints. Even if the risk / reward is suboptimal