Durability is at the top of the list. Where overhead obstructions are common (like here in Washington state), a fiberglass antenna is a liability. They are brittle (especially in the winter), and it is not uncommon to have them shatter when they strike an obstacle. Once they crack, they usually break the wire winding inside, and can damage your radio if you transmit with it. They don’t even need to crack to break. If they take a hard hit, or scrape against rocks enough, the wire can become damaged, and again cause radio failure. The bad thing is, you don’t always know when they are damaged, and keep using  it anyway…. then it’s too late. Unless you keep an SWR meter in-line all the time, you would never know until you fried your radio. People have even fried them twice, because they didn’t know what caused the radio to fail the first time, and hooked up the repaired radio to the same damaged antenna. Only then did they suspect the antenna. (I always do, and ask my customers to be sure it’s okay).