Metal servo arms are used when you need a long arm application, like a 3D airplane, to avoid flexing as stated above. Jet models are different, you should always strive for a short servo arm and in this case the standard servo wheels will do fine, and anyway, if you don´t get peace of mind
by using plastic horns on the airplane, you shouldn´t have it too using servos with plastic cases!
Heavy duty ball links are also used without problems in any particular aplication that needs some
angular movement on the linkage, otherwise, du-bro 4/40 metal clevises are the norm.

Best regards, Enrique