You're comparing apples and oranges. Mellanox has Ethernet switches with 300ns L3 latencies -- far lower than their broadcom counterparts. So it's not an Ethernet limitation, but a broadcom limitation.
That means that these switches, while fast, cannot check the packets for correctness (they don't have the full packet). That they will have "aborted" packets. That in some important ways these networks have the problems of the "half-duplex" networks of old.
Broadcom focuses on features for packet transmission. That means these Mellanox switches are pretty much restricted to situations where you want to have a set of servers on a single network segment and nothing else (not even an upstream connection). If that's exactly what you need, great. But mostly you're going to need more.
Your information may be old; Mellanox has pretty much the same feature set as Broadcom now.
Also, you can run cumulus on the switch, which is pretty awesome.