I, on the other hand, use a NDI PoE PTZ camera using Ethernet, and love it. True there will be a slight difference in latency (vs SDI), but with a properly managed (isolated) video network, all is well (for us).
The bigger issue for Gina is mixing video processing data baths (is this case network card, and USB). that can get tricky to manage with the expected/natural processing latency difference. And then, just as important, is whether the PC is capable of handling processing both video input decoding, and then encoding for output (a little hardware resource monitoring in order). If the video latency difference is consistent, that is good news, and if I recall someone else's comment recently there is now (more recent OBS Studio versions) an option to adjust latency per video source?? In which case, slow the USB camera down (add delay) to match PTZ. Aaron is fine with using person clapping or similar, I'm a little more OCD and prefer to use something like
https://streamgeeks.us/fix-audio-sync-issue-in-obs/ to get sources in sync (ie point both cameras and a microphone at laptop/tablet playing video.. and Record in OBS Studio. you should be able to see the sync differences and adjust accordingly.
The other consideration, though a challenge for budget, is moving to cameras all using same transport (in this case Ethernet, or whatever ) ... with Power-over-Ethernet PTZ cameras being single cable (video feed, control, and power), and if you have local networking expertise, so much cheaper, easier, more flexible, etc
And 'big' in this case can have multiple interpretations, so best not to use in this context (physical device size [not really relevant to anything in terms of video], resolution? etc). What would be relevant is knowing if the USB camera is say 720P and the PTZ is 1080p or 4K ?
And the network layout could certainly come into play. Is the livestream/video network isolated from other traffic (ex guest WiFi)? is the Ethernet switch (PTZ camera and OBS Studio PC connected to) some PoS like TP-Link or other low-end consumer brand, or is it capable of low latency/jitter processing of video traffic? etc. With limited budget, I used a spare consumer router I had laying around to act as firewall between livestream and office network (only used for single upstream connection to office LAN/Internet), then we got a Netgear business class managed PoE switch (VLAN capable) for OBS Studio PC and PTZ camera(s) and nothing else is allowed connected to that environment. The office is likely to be upgraded in near term (real firewall, VLANs, etc), so consumer router/firewall will most likely go away.. finally.
I also setup for mobile device being able to stream video via NDI over WiFi ... 'similar special interest' [think Palm Sunday procession outside Sanctuary] but we've never used it... maybe one day
