What we call "low-performance" NICs have been observed by users, in loaded systems, to consistently lose packets, with corresponding poor data throughput performance. These NICs don't use descriptor rings and DMA, but for simplicity, instead attempt to buffer the entire packet in a (usually limited) on-chip buffer area.
Unfortunately, these low-performance NICs, because of their low cost and size, are very attractive to board designers. Examples of these (usually older, obsolete) NICs include:
On a fast (e.g. 2 GHz) lightly loaded machine, these low-performance NICs can function adequately, without packet loss.
However on a slower (e.g. 100 MHz) machine that's CPU-bound with applications that may increase the scheduling latency of io-pkt*, packet loss during receive can often result because the limited hardware buffer overflows.
You shouldn't use these NICs where you need high-performance data throughput. You should use them only for low-cost debug and diagnostic ports, which are often removed for production versions of boards.
If you're using NFS used with one of these low-performance NICs, you can get a great improvement by using the -B4096 or even -B2048 option to fs-nfs. Qnet in QNX Neutrino 6.3 and later generally automatically goes into "windowed mode" with these NICs to try to avoid packet loss.