Tatsu won’t beat the 45 degree air fireballs. Tatsu assist loses to those and Magnetic Blasts depending on the spacing. Low fireballs are often used by players newer to Morrigan, but she has much more annoying patterns.
I think Chris G’s team is close to optimal, and her most optimized teams are Morrigan/Vergil/Doom, Morrigan/Doom/Strider, Morrigan/Dante/Strider, Magneto/Morrigan/Doom. It depends on the matchup, but I think those are the best Morrigan teams for her support wise, ie Magneto being a battery for her and setting up TACs into Doom, Vergil giving her practical ToDs off any kill. While Jam Session/Vajra/Hidden Missiles are her strongest assists and Doom providing TACs and command grab setups, Dante providing DT DHC off invincible reversal Shadow Servant or command grabs. Strider offers only Vajra, but he comes with the derp that is XF3 anchor but I don’t think Morrigan 2nd Vajra 3rd is optimal due to damage issues.
I think just Morrigan overall as a character is very annoying to deal with, Astral Vision giving her free wins vs anyone not Vergil, her fireball angles hard to interact/deal with, her complete safety, and the fact that it’s hard to rush her down when she has shell kick to land with. She’s a very strong character and made scarier with Missiles/Vajra.
As for characters that face off well vs the Morrigan/Doom combination, I like the following assisted(preferably with Vajra or another strong neutral assist):
Chris, Hawkeye, Dante, Vergil, Dorm 2nd(with momentum)
Maybe: Deadpool with Vajra(not sure about this), Viper with Vajra, assisted Yolo Captain America might give her problems, Nova?
On another note, Morrigan/Vajra is a different matchup, and is a much worse matchup for most characters vs Morrigan than Morri/Doom due to the losing of the ability to stall at super jump height vs her. Vajra is really stupid good for her when she activates astral vision. Missiles/Jam Session provide better rushdown, but Vajra makes astral vision broken.