My current place of employment has Comcast. We have a major outage every couple months during which all work ceases. I rely on an Internet connection for everything I do professionally so I stay away from cable because stability > speed.
There's really no point in saying a nationwide company is reliable or not because it's incredibly region-specific. At home I've never had connection issues, I generally get the speeds I pay for, and I've never had any billing issues. But you ask someone who lives 100 miles in any direction with Comcast and they'll have a completely different story.
(this outage: https://www.wired.com/story/how-a-tiny-error-shut-off-the-in... )