I'm seeing that low-end bikes have freewheels while everything else uses cassette free hubs. Are freewheels that fragile? I understand that the bearings on the free hub are more indestructibe, however are all the low-end bikes with 7 speed freewheels really that weak that they should be avoided? Giant, Specialized, Trek, etc. all have bikes with freewheels. Are those bikes just junk?