What was IS thinking making the Muspell units Playable and other possible units to come.

Why's it such a huge problem that they're becoming playable?

Well, we will probably get more info on why the Muspell units will be recruitable most likely in the next chapter in 40 mins

I mean, all of them have shown that they're not totally evil. They literally turned against the guy who burnt down innocent villages, intended to sacrifice children to his Dragon god and even his own children if he felt the need to. They're definitely not irredeemable. I think this was a good decision. Regarding Bruno and Veronica, they haven't explicitly resolved the feud between Veronica and Askr, but she has helped her enemies and defied the person who shared the common goal of destroying Askr. I wouldn't be surprised if her and her brother were to become allies soon, too. Loki, however, is really shaping up to become the next major antagonist. I wouldn't count on getting her anytime soon.