First of all, its free and its fun, so not too many complaints!
I use this site to track my son on ferry flights. 29Jun from KOSH to KBVI, it dropped him - over Lake Michigan! only a short time into the flight. Never picked him up again by tail number N191ND, but caught him several hours later on the KBVI map just before landing. Eariler that day, it correctly tracked a flight from KGFK and a return for a repair. But when he left a few hours later to KOSH, it tracked ok in real time, but retained his original departure time and did not show the whole flight in history after completion.
Also, on ferry flights to/from PHNL the three I have watched always drop off and show arrived about an hour from the destination (most recently N291HD, 21Jun). A bit disconcerting, but I’ve come to expect it.