Wireshark has bugs, yes. Mostly in the dissectors and in the UI. But the packet capture itself is through libpcap. Also, to point out the obvious: pcap viewers in turn are auditable if and when necessary.
SPAN ports are great for network troubleshooting. They're also nice for security monitors, such as an intrusion detection system. The IDS logically sees traffic "on-line," but completely transparent to users. If the IDS fails, traffic fails open (which wouldn't be acceptable in some circumstances, but it all depends on your priorities).
No, really, I get where you and your parent are coming from. It is a low probability. But occasionally there is also thoroughly verified application code out there. That is when you are asking yourself where the error really is. It could be any layer.
Parsing the pcaps is much more prone to bugs than capturing and storing, but that’s easier to verify with deserialize/serialize equality checks.
And how did I get that figure?
I'm going to fold pcap overhead into the per-message size estimate. Let's assume a trading day at an exchange, including after hours activity, is 14 hours. (~50k seconds) If we estimate that during the highest peaks of trading activity the exchange receives about 2M messages per second, then during more serene hours the average could be about 500k messages per second. Let's guess that the average rate applies 95% of the time and the peak rate the remaining 5% of the time. That gives us an average rate of about 575k messages per second. Round that up to 600k.
If we assume that an average FIX message is about 200 bytes of data, and add 50 bytes of IP + pcap framing overhead, we get to ~250 bytes of transmitted data per message. At 600k messages per second, 14 hours a day, the total amount of trading data received by an exchange would then be slightly less than 710GB per day.
Before compression for longer-term storage. Whether you consider the aggregate storage requirements impressive or merely slightly inconvenient is a more personal matter.
0: https://robertwray.co.uk/blog/the-anatomy-of-a-fix-message
It could be funny though because you could be able to bump up your archive storage requirements by changing an IP address, or have someone else do that. But that's life.