I heard about this last week and tried it with my Sony α6100 on Windows.
All of these sorts operate over PTP, roughly saying: “camera, please take a photo of what you see (but don’t save it to the SD card)” followed by “camera, please hand me that temporary file you just created”—30 times a second, and then serves the JPEGs in succession as an MJPEG. I know this is how it works because I was figuring out how this stuff worked a month or two back (Wireshark and all) with a view to creating a way of using it as a webcam.
My camera only captures at 1024×576 this way. This is disappointing. I suspect that this resolution may have been chosen so that it could still serve 30fps over USB 2 (480Mbps, and 1024 width × 576 height × 24 bits × 30 frames = 405Mbps, though in practice JPEG compression is used so it’ll never be anywhere near this). I’m not sure how fast it actually does transfer over USB, but the one time I imported my photos to my computer over USB, it was vastly slower than just taking the SD card out and putting it in the Surface Book’s reader, which let me copy multi-gigabyte video files off at 2Gbps. (I’m still marvelling at that speed every time I do it.)
Despite that low resolution, it will still look better than almost all laptop webcams at 1280×720 or even those at 1920×1080, because of superior optics.
Another thing to be aware of is much higher latency—I observed a delay of 150–200ms, which is extremely not great.
So my conclusion all up is that, at least for the α6100, this is not suitable for serious use. I guided someone last week to using https://ptpwebcam.org/ on their Mac last week (having come across it in my earlier research), and he reported getting a much better resolution with his Nikon camera than with his Sony camera. Not sure about latency, I didn’t ask.
The alternative in all this is HDMI capture cards. Unfortunately, decent ones are moderately expensive—but then, you bought a DSLR or mirrorless camera, so you’re clearly not afraid of spending money, even if it is a pity that you have to spend more when USB really should be quite capable. I haven’t acquired one yet (I’m in India at present, and the options available are extremely limited, though curiously far wider today than they were two months ago, when there basically weren’t any below US$200), but from my research I say this: do not get one of the cheapest ones that advertise USB 2.0 operation, 1080p and maybe mention 30fps. It’s genuinely impossible for them to actually achieve this as they should: 1080p of 24-bit colour at 30fps is 1400Mbps, which is treble the theoretical limit of USB 2.0, and I’m not sure what the practical speed limit of USB 2.0 is, but it’s typically quite a bit lower than that. So in short they must be lying and sacrificing image quality and/or frame rate. Reviews of such capture cards are pretty consistently a mixed bunch with some saying “sure, it works fine” and others saying “no, this is terrible and unusable, don’t buy it, it was a waste of money”, sometimes with proper technical detail of where it fell short. Instead, seek as a minimum one that advertises USB 3.0, 1080p and 30fps (and make sure it actually says all three of these things, it’s muddy ground where there seem to be some that don’t say or consistently achieve 30fps). There are still various options a lot cheaper than popular known-reliable units like the Elgato Cam Link.
I’m not sure what latency is like if you use an HDMI capture card, but I’m guessing it’ll still be enough (2–4 frames at 30fps) that you might want to fiddle with latency compensation if you’re using something like OBS Studio, to align your audio and video sources.