I'll confess to buying a carbon ultegra road bike from Amazon. I know this likely is not a popular decision and trust me a really really wanted to support the LBS, one specifically but they couldn't come close to the price with competing manufacturers.
As for the Amazon process it was pretty clean. Bike delivered the next day from order. Even extended 18 months interest free. Really hard to beat. Fingers crossed though that no issues outside return window as I know I'm looking at no support at all. This is probably the big rub buying from Amazon. You get nothing with it. Luckily I fancy myself fairly handy wrenching so short of some catastrophic issue I figure I'll get it worked out.
Some Amazon bikes do have free returns, some dont list it so and I suspect they may give you grief if you try and return. Just be very clear on what your expectations are. You likely gain cost advantage but your giving up support post sale for sure. This one still troubles me still but dam the price was impossible to pass on.
Good luck and good riding.