I am not well versed in materials but I have been painting for a while, I can say that synthetic is fine but it is somewhat easy to tell if something is junk by looking at it. Really lousy brushes will have thick, stiff, plastic bristles that aren't good for anything but a child to slam paint onto a macaroni Moses.
I pulled out a "quality" brush that I'm not happy with: the handle says 12 Pro Art 4001 China. I'm not proud of buying a brush that was probably made by slave labor but I probably bought this in college years ago when there was a firesale on art supplies in town. I said that stiff bristles are really bad, this one has bristles that are so soft that it's almost impossible to use. It's so soft that it can't be "trained," it prefers to bend or sag in the wrong direction rather than make a consistent tip.
Buy some brushes that don't cost $66 and test out your preferences, in my mind it's about finding bristles that are the right length, softness, and cost. I have a 2 Royal Gold RG 585 brush that I am very fond of even though it's a a full 1" long (rather long for a mini painting brush, it would scale up to a pin-striper's brush on hot rods, motorcycles, and such). I don't use it much but when I do it does justice for me.
Don't be afraid to buy the cheaper brushes at art supply shops, so long as you can inspect them before buying (never, ever get brands designed to be sold to children). I have several brushes that are trash but they're the right size and still work fine. Half of brush work has, for me, become slowly twirling the brush around in the paint to get the tip into the right shape, similar to how vital it is to get the right mix between paint and water.