Realtek itself (Questionable quality on a good day)
The implementation of Realtek by the ODM/OEM/SI into whatever part is being shipped, which given Realtek is the defacto "budget" option for networking, it's often done as cheaply and shoddily as possible, even if the chip itself actually isn't crapware.
And the USB interface as you point out. There's a whole rabbit hole that I'm unfortunately all too familiar with when it comes to diagnosing and dealing with USB. PCIe passthrough via a USB4/TB combo chip isn't as reliable as just using PCIe directly, but it's still better than a non-pcie passthrough usb interface.
Realtek itself (Questionable quality on a good day)
The implementation of Realtek by the ODM/OEM/SI into whatever part is being shipped, which given Realtek is the defacto "budget" option for networking, it's often done as cheaply and shoddily as possible, even if the chip itself actually isn't crapware.
And the USB interface as you point out. There's a whole rabbit hole that I'm unfortunately all too familiar with when it comes to diagnosing and dealing with USB. PCIe passthrough via a USB4/TB combo chip isn't as reliable as just using PCIe directly, but it's still better than a non-pcie passthrough usb interface.