Wikipedia has a nice list of previous Comodo malfeasance: https://en.wikipedia.org/wiki/Comodo_Group#Controversies Highlights include allowing fraudulent certs to be issued for Google, Yahoo, and Mozilla; working with a malware company; and trademark shenanigans with Let's Encrypt.
>working with a malware company Why is this a bad thing? A cert says "Yes, person x REALLY IS person x, and I can prove it mathematically." It doesn't say "Person x is trustworthy enough for me to vouch for them."
For malware concerns there's also Comodo's relationship with PrivDog. I'm not clear on whether PrivDog is deliberate malware or just incompetent insecure software.