We originally had Verizon and they were good. Very fast and reliable. Frontier bought them out, and during the switch over we had lots of problems. We went from having no outages to weekly outages. Whenever I tried to call Frontier, the customer service was terrible. Hours on hold to speak to someone who you couldn't understand or would basically say there was nothing they could do. All the neighbors had similar issues. Eventually, we switched to Charter. The overall service has not really been more reliable, although they almost always send someone out same day or next day, even if it's not their fault.
From talking to neighbors, Frontier's outages have calmed down, and they are pretty reliable again. However, I have heard their customer service is still terrible. Still, though, I am considering switching back.
All that said, my inlaws who live about a mile away also have Frontier, and they never had issues. I live in an older neighborhood, and in general we have more outages (not just Internet, but also power) than the others in my area. It could just be my neighborhood that has had so many problems.