Tail drop is the default congestion response. When a queue fills, new packets drop until space becomes available.
Tail drop problems:
TCP synchronization. Packets from many flows drop at once. All reduce windows together, then ramp up together, causing repeated overflow.
Full queues mean high latency. Packets wait behind many others.
Bursty recovery. Queue oscillates full to empty. Utilization swings dramatically.
No differentiation. All traffic drops equally. Voice drops as readily as bulk.
Tail drop works when queues rarely fill. On congested links, you need smarter mechanisms.