How do 3D printed objects compare to ones made with coventional manufactuing techniques?

I am a manufacturing fine jeweler and 3d printing has swept my industry. Most of the jewelry you see on TV was 3d printed. Our printer is the Formlabs 2, an entry level SLA resin printer of fairly high resolution. We use castable resins (which are gaining in reliability, thankfully) and lost wax casting. There are some direct-to-gold processes but they are 1/4 million dollar machines. The raw materials are gold powder made to be finer than talcum powder and it costs on the order of $10K/ounce, which is prohibitive and not likely to change much for some time. My printer makes anything I need to a level of precision that is industry standard. The best part is that the 3D graphics part provides a level of precision that used to be only found in the highest jewelry houses with prices to match. Right now I have a job that needs 140 stones arranged in rectangular shapes, which are then curved. Old school was to drill them by hand, with varying results. Using the array command it takes a couple of minutes to complete them all with perfection. Cool beans.....using Boolean I can drill star shaped holes, if I want to. Try THAT with a flex shaft!

Your question: steel tools are more likely forged, which means a giant press mashes them out of steel stock. That gives superior results no matter whether the competing process is 3d print or sand casting or whatever. Forging is just how they are made. I can use around 15 different resins in my printer. I use the one that suits my business. But I can easily print a smart phone case or a pair of sunglasses. I forget the details, but my capacity is something like a 6 inch cube - anything that fits inside of that. When you get to long run production of products like smart phones - hundreds of thousands if not millions of units, then you want conventional processes like injection molding. That is pennies per part - my resin is $299/liter. But you can't beat it for prototyping. There are jobs that I used to take a couple of hours to do that I can draw in 10 minutes, literally. Send it to the printer and while it is running (2-3 hours, typically) I am doing other work. And short run production. I lately printed 85 identical, tiny parts. Perfect and identical. Old school for that would bring nowhere near the quality overall and take 10x as long, hands on.  Seriously cool stuff. I love my printer.

It all depends on how it's used and the technique used to make it.

There are many different 3D printing processes, using many different materials.

Home / DIY printers are all still in general plastic or resin based so far, but industrial ones can produce solid metal parts.

Some can produce parts that are just as strong as if the item were a traditional metal casting.

Different colours or different grades of similar materials can easily be done with the plastic extrusion type printers, including mixing hard plastics and rubber/elastic sections plus clear sections in the same thing.

Some places are making machines that can print things with electrical conductors included, so like circuit boards. Resistors may be possible, but semiconductors need very different processing techniques and I cannot see anything of integrated circuit complexity being made within the same machine for many decades, if ever.

Including a parts supply and "pick and place" system to allow ready-made complex components to be "built in" to a thing while it is being printed is another matter and definitely possible.

Foods are another thing again.

For many types there are no technical problems of forming shapes or layers, but lots of foods have incompatible cooking or processing techniques - eg. you cannot "print" ready-cooked dough or batter for bread & cakes etc., but a lot of other things that you would want on/on the bread or cake etc. are not compatible with the cooking of the dough or batter.

A machine to print things like that would need a separate oven section etc., and a simple thing like a basic sandwich could take several hours to create..

And, it has store all the possible ingredients and be able to to totally cleaned & sterilised to prevent anything decaying.

Other things like chocolates could be far simpler; chocolate shapes or hollow shells could be printed with extruding machines similar to existing plastic extruding ones & fillings or flavourings added in the same way.

For mass production of many things, existing processes are simply faster and cheaper - but 3D printers are unbeatable for prototyping and small quantity production, or where each part needs to have sizes or details adjusted.

I don't see food production being for other than specialised items or business prototypes & small run samples, or simply as a novelty.