I bought both my mountain bike and road bike online (Fezzari and Canyon, respectively). Both were good experiences. For the Fezzari, I knew what I wanted fit-wise based on my older bike. The price was great, and I was able to make a few component changes (tires, dropper, brakes). The process was pretty easy and took a couple emails and phone calls, and I had the bike in about 3 weeks.
Buying the Canyon was much more like ordering off Amazon. I saw the bike I wanted in their outlet store (very slightly used) on Friday afternoon, and it was on my front door on Tuesday afternoon. No human interaction at all.
I prefer the Fezzari model to the Canyon model because being able to customize the bike is a huge advantage.
While cost was a big driver for both, there were a few more advantages to buying online. As a taller person, it is harder to find bikes in my size in the shops, so there isn't much to test ride anyways. I also really don't like it when people are trying to sell me something. I don't enjoy the pressure, and there often isn't much they can tell me that I didn't already read on the website.
For both of these bikes, I had a very strong idea of what I wanted. I understood the geometry and how that compared to my previous bikes. If I didn't have that background, I would have been more hesitant to buy without a test ride.
Both were ready to assemble. I have been wrenching on my bikes for a long time, but they would have been easy for someone without much background.
