When I started a thread a while back about the various Shimano groups, I picked Tiagra/Deore as my favorite because it was the lowest level where it seemed no compromises were made for price over durability and longevity, but no extra cost was added for weight savings. Aluminum is used for the three big chunky parts (the knuckles and front plate), and plated steel elsewhere.

The Claris RD (two levels down) has only the rear knuckle made of aluminum. The front knuckle is plastic and all the other pieces are painted, not plated steel. You can find similar compromises throughout the groupset. This is not a showstopper! Paint and plastic can last a long time, especially if you don't leave it out in the sun and rain. But it will, ultimately, wear out or rust sooner.

For a comparison in the other direction, the Ultegra RD (two steps up) has the front knuckle made of carbon fiber reinforced plastic, and all the other big pieces are aluminum. The pulleys have ball bearings.

Because I'm becoming a nerd for this stuff, I'd be really interested to find out which pieces are cast and which are forged, and details of the plating (nickel? chrome?) and painting (enamel? powder coat?)... but the specs don't say.