They use cast aluminum for their Trigger Assembly Plates (Different than MIM). Cast aluminum is on par with steel parts in strength and rigidity. One of the main reasons they use it with their Police model shotguns is because of the strength and it's longer service life compared to the synthetic Trigger Assembly Plates.
Items machined from a Billet is stronger than cast materials.
Aluminum billets are either forged into shape or formed using continuous extrusion, either of which process produces a finer, stronger, more uniform grain structure than casting does.
There are different grades of casting as well. The aluminum trigger guards Remington uses on their shotguns and rifles are low quality. It doesn't really matter because both polymer and cast aluminum guards Remington uses get the job done under normal use. I wouldn't pay the price Remington charges for their cast guards.
The steel guards Mossberg use on it's shotguns and rifles are the best.