When your client wants you to build complete amazon in a week.

Oh come on, it is just some buttons. Cannot be that hard....

My boss wanted to create excel-but-better for web in three months 😓

I would say the codebase of Amazon is super big and well structured. Unlike spaghetti codes... It goes upon proper reviews and maintains lots of standards. Hence, making a small scale website may be simple, but building cross region, multilingual, cross browser will be difficult..

Well Amazon may be a masterpiece of engineering, but it is surprisingly "cheap"-looking and difficult to use in many places. I frequently drift into the let-me-please-fix-this rant-mood when using it for longer periods...