Here's an idle thought: what are the differences between the three methods of sintering, casting, and metal injection molding?

I have a vague notion of how they work- all basically molten metal put into a mold to achieve the final shape. But there has to be more to it.

Do you even hot isostatic press, bro?

In before MIM in airplanes.

Casting requires molten metal.

Sintering is metal powder only.

MIM is metal powder + flow additive/binder.

With proper QC and heat treatment, there's no problem.

Issues arise because there's little or no QC, which often goes hand in hand with MIM and sintering, since they're usually employed as a cost cutting measure in the first place.

I'll take a casting over MIM.

For me it depends on what do you are doing, however most of the time I would just go with good old casting.  especially with basic iron and aluminum.

If you cut enough corners you can fuck up almost any process. Add little or no QC into that and the high percentage of bad parts ship to customers.

All the processes you stated above can be fine for their intended and proper use, as long as it's done properly and with enough QC to prevent bad parts from leaving the premises.

Powdered metal is actually capable of superior metallurgy strength if done correctly.   I think the key is cost, it's done as a cost cutting measure in most consumer products so it is done in a manner to maximize cost savings. Cost cutting is not also how you get maximum quality.   

I work with cold work tool steels....swiss cold work tool steels.  Bohler makes popular cutlery steels as well, Elmax being one that I believe is powdered. They have an entire product category that is powdered metal and the properties of the steel are fantastic. They are superior to any tool steel created with traditional wrought processes, it's all about grain size, mixing and carbide distribution. 

That all being said most consumer level powdered metal, sintered, MIM is shit, or at the least inconsistent.