Generally speaking - there aren't many areas where the finished product using 3d printing is better than more traditional methods. The main advantage of 3d printing is the flexibility. Injection molding will pretty much always give you a better result, but you'll be able to go from idea to physical prototype much more quickly with 3d printing - not to mention that your average person isn't going to be able/willing to set up an injection mold in their homes.

In industrial use (so not consumers) the main use for 3d printing is indeed rapid prototyping, 3d printing doesn't really scale all that well to mass production, but if you're going to be iterating on a design several times before getting something you're comfortable with, then it's a great choice

You're not going to be able to do an FDM print without there being layer lines and without there being a plane in which the part is more fragile thanks to layers being easier to delaminate from each other than to break. A resin printer can more or less avoid these problems, but you'll need an expensive engineering resin to get the same strength. None of the problems affect everything you might ever want to make, though, and there are plenty of things you might only want one or two of, so obviously would never machine a mould for.

It depends on how you define quality.

• If you define it by general shape and tolerances, 3d printing can(!) achieve a comparable quality.
• if you define it by it's surface finish, SLA prints can but it's nearly impossible with FDM (at least currently).
• and if you define it, by its tensile strength or mechanical properties injection molded pieces wil probably always have an edge over 3d prints because of their anisotropic behaviour (meaning the piece can endure forces differently depending on the direction)

And of course as already mentioned injection molding is a much better fit for mass production.

It's not the quality that's a problem, it's throughput. In the hours it would take to print a part, you could get hundreds of an injection molded part.

They can be, it largely depends on how well calibrated the printer and the print settings are. 3D printing definitely is vastly superior for low quantity production or prototyping (due to the costs of getting molds) but it generally speaking isn't the best for mass manufacturing.