When the sum of inputs for a particular output destination exceeds the capacity of that output, then switch gets congested.

Since switches have large number of inputs (which may regulary have packets destined for a single output), they likely sites of network congestion

To reach the an equivalent level of performance, output buffered switches require more buffers and a faster switch fabric than input buffered switches.

Congestion will fill all buffers. Dead time or spare capacity is required to empty full buffers.