Often the devil is in the detail. A 105, or what ever, often has half the components exchanged with something else. Jagwire cables, tektro brakes, fsa crank, kmc chain, sunrace cassette, no name hubs, etc. Often its only the shifters and derailleurs that are actually 105. The rest may or may not work very well. I would take a full Tiagra bike over most hodgepodge "105" bikes.