If I were to say one thing about Capella, it's not for the disorganized.
I got my MS and started a doctoral program at a brick and mortar school 20 some years ago so I have some basis for comparison.
Most of the faculty are very good and stay engaged in the courserooms. I had two instructors who not nearly as engaged. While disappointing, it wasn't out of character with my previous grad school experiences.
I never had a problem with learner support, but I went to a large midwestern land-grant school as an undergrad and was used to the idea of taking the bureaucracy by the horns. They were always polite and did what they said they were going to do. My employer paid for the lion's share of my tuition, but it only covered 2 quarters per year so that's all I ever enrolled for. I got a bit tired of getting called when I didn't register for courses to convince them I really did know what I was doing.
WRT the dissertation process, it's ambiguous wherever you go, that's why getting a Doctorate is hard. So much of the frustration of the dissertation process comes from having an adviser/mentor that you don't get along with or just don't communicate well with. Choose carefully and your life will be much easier. Try finding someone who's had students successfully graduate. The proof is in the pudding. Besides basic interests, I only nominated mentors who had their mentees graduate on a regular basis.
I would recommend Capella more to folks who have jobs already and can't be running off to classes several times a week. There is an unfair stigma to online only degrees. Capella will make you work for your degree. It's no diploma mill. That being said, it's not for everyone. There are many distractions when you do your schooling from home. If you can't block out extraneous signals, you're going to have a much harder time.
You can get the minimum out and cruise at 'B' level work, but you'll have a hard time passing the Comprehensive exam and an even harder time getting it together for the dissertation.