My 2 complaints about amazon (from a shopping user experience) are the push for prime and the items that qualify for free shipping. Push for prime seems to come at every stage of using the mobile app. Start it up, it asks. adding an item to your cart, it has a prompt about prime. Picking shipping method a few times it has defaulted to signing me up to a trial of prime. Right before final purchase, another prompt for prime. Yeah, I get that prime is a pretty sweet deal, but I don't need it. I am not a "last minute shopper" so prime doesn't offer much benefit to me.
The other annoyance is finding items that qualify you for free shipping. I have over $65 in my cart and I need to add $6 and change to get free shipping because some of my items don't qualify. Best I can tell, there is no magic button I can click on to ONLY show me qualifying items.
But apart from that, Amazon is pretty good. I've had a few bad experiences (damaged goods; parcel never arrived; parcel missed target date; parcel had a target date, Amazon updated target date to unknown; Item was ordered and paid for and later refunded because they lacked inventory). BUT overall, the experience with Amazon is good. Web interface is a lot nicer to work with than the phone interface too.
Also, the gift wrapping option from Amazon used to be better. I remember ordering things and having it nicely wrapped in nice wrapping paper. Now every parcel I've had gift wrapped is shoved into a gift bag. It is a nice gift bag, but the wrapping paper I felt looked nicer. Not as good for the environment mind you - gift bags are reusable, wrapping paper is not.
Touching on Eric's comment about companies failing with mobile apps - another thing I've seen some companies do is have their mobile app be a glorified web browser. (NOTE- this applies to Android only as I have an android device... may be different on iPhone). I hate it when the app is just using the system webview to present a specific webpage inside the app. It is clunky, buggy, and shows to me that the enterprise doesn't care about its mobile presence.
I think the market is now demanding that a company has a web presence and where possible, delivery options. Walmart has done a poor job of this in my area - I can order my stuff, pay to have it shipped to my door, and in 2-3 weeks I get a call from the delivery company telling me they don't deliver to my area and I can come pick it up myself at their location which is about 5 minutes away from Walmart. I've complained to both Walmart and the delivery company and now both are ignoring my calls and emails. BUT Walmart offers parking lot pickup and they do a 5 star job of that in my area. Just need to do a 40+ minute drive to get there and 40+ minute drive home after.
You also made a reference to Wayfair. I've ordered stuff form there and it seems to be hit and miss on delivery. Some stuff comes very quickly, while others come incredibly slowly. I had placed an order and it was going to be a 6 month wait. I cancelled the order and changed it to a similar product and had a 4 month wait instead. This is annoying, but also I am excited about the product so I can be patient.
I have not used the other company you indicated (build.com). Living in Canada, I am not certain they would be able to help me much. Plus, my house is pretty new - finished being built in October 2020 with the garage expected to be completed in the next few weeks. Minor renos required still, but I am hoping I can tackle some of them myself as contractors for the remaining bits have come back with really high quotes.