TBF drop means the user looses throughput for a while (few seconds), but that’s usually enough to stop the ftp transfer or prevent the webpage from being displayed fully. So yes, in the end, a tbf drop is impacting the subscriber experience.
It is mostly due to radio problems (interference, coverage, mobility…), but other reasons can happen : PCU software failure, TCH traffic preempting the TBF’s radio timeslots, or abis/ater failures. The failure of the Gb interface does not impact the TBF drop (but the Gb should never fail, anyway).
If it is a DL drop, the PCU will establish a new TBF for that MS in order to retransmit the data. But as I said above, it is probably too late for the MS : the webpage or the download is already stopped.
If it is a UL drop, the MS is not obligated to establish the UL TBF again. It is up to the subscriber it establish it again.