I think JA is fine as a one-time source for a stone and ring... But that's really not the way I advocate buying a ring. Where the JAs of the world tend to fall down is on service. They're just not set up for long-term relationships with their clients. The nature of e-commerce is transactional, not consultative, so the relationship just doesn't get built.
When you buy an engagement ring, unlike virtually anything else you will ever buy, you will have it FOREVER. Someone will wear it, every day, for the REST OF HER LIFE. There aren't a lot of products like that. And, like anything you own long term, it will need service and maintenance. So you need to have a relationship with someone you can trust to take care of that stuff.
I've said this in this sub, and elsewhere, many times. Your jeweler shouldn't be the equivalent of Amazon. Your jeweler should be the equivalent of your lawyer, your doctor, or your accountant. At the end of the day, a diamond is a commodity. You can get one in lots of different places. So what you're paying the jeweler for is professional expertise, advice, and guidance. You're paying the jeweler to stand behind his product, and help you care for it, forever. It's a meaningful commitment on behalf of the jeweler, and it's one I take very seriously.
So, if you don't know jewelry, KNOW YOUR JEWELER. You just can't do that with JA, BN, or any of the other big sites.