HD-PVR's and other USB2 capture devices are pretty vastly inferior to a decent internal capture card, primarily because of interface -- in order to fit HD video through the USB2 pipe (relatively small), the data must be significantly compressed using a dedicated encoding hardware chip in the box -- the compressed stream is sent over USB to the host, and for capture purposes this can be great, as encoding is a processor-intensive task so it being pre-encoded means you can just save the data directly to disk. But if you want to stream, or mix the video at all (as OBS does), it has to be decoded (a relatively easy task) so that the raw video data is available to be used (turn it back into just normal pixel data), then it can be mixed and *re-encoded*.
Unfortunately, this means you're losing video quality in that initial encode process (it's "lossy compression"), and the extra encode->transfer->decode process takes more time, so there's a lag to those devices (the HD-PVR's is usually upwards of a couple seconds, IIRC). That means that the video captured will be behind any audio or webcams on the system, which are designed to have lower latency -- so you end up with worse footage, and it's behind everything (so syncing it with other stuff becomes tricky and expensive, resource-wise).
An internal capture card on a PCI-E interface, meanwhile, doesn't have to do any of this, as the interface is fast enough to handle the data -- in this respect, internal devices have always had a significant advantage over externals like the HD-PVR. USB3 similarly have this advantage, though some controllers have trouble handling that amount of data.
However, it goes beyond just that -- The way that drivers are engineered for the devices matters as well. For video capture, you want devices that use a software interface called "directshow", which is just a kinda-uniform interface to help programs know how to handle data from different video devices. It's not really built to deal with pre-encoded datastreams, though, so to use them you need software that understands how to talk to such devices separately and specifically. Some manufacturers provide effectively a wrapper that handles the device-specific talking-to ("Stream Engine" for LGP's, for instance), and presents a directshow interface that OBS already understands. That doesn't eliminate the lag, but makes supporting it easier.
Some vendors, though, only provide a way to preview the stream in a window, and don't provide software developers (read: OBS devs) enough information to know how to talk to, and understand data from, a device. That's the approach Hauppauge took with their HDPVR, which is why it's inferior and tricky to support: Doing so will require reverse-engineering how the vendor software talks to the device, which is a lot of effort better spent (from the perspective of someone who *doesn't* have one) on other improvements.
Hope that clears things up.
