No. No protocol limitation. Not at all.
This is strictly about the chipsets used.
Original Alpine Ridge TB3 controllers had that internal bandwidth limitation. They were just slow when it came to PCIe.
Since Titan Ridge TB3, this problem is gone. Titan Ridge can reach the full bandwidth of its x4 Gen 3 PCIe connection (i.e. its held back by the connection to the GPU). Same for Goshen Ridge and Maple Ridge TB4 chipsets (which are just USB4).
The only reason the UT3G and UT4G are faster is, because they use the ASM2464 controller, which has a PCIe x4 Gen 4 port, so it can exceed the former 32 Gbit/s limit with any host that can also exceed that (which is anything newer than Intel 11th gen & Maple Ridge).
Intel has since also started selling new controllers (Barlow Ridge) that also have x4 Gen 4 ports and come also in a 80G variant. Barlow Ridge are also USB4v2, which would allow them to leave a protocol efficiency limitation of TB3 & USB4v1 behind, that has made PCIe through them less efficient than native connections (what causes you to only see ~ 25 Gbit/s of usable bandwidth of an actual 32 Gbit/s PCIe connection).
This limitation still exists in all current CPU-integrated AMD & Intel USB4 controllers and the ASM2464 as they are all USB4v1 though (they just do the same with a ~ 38 Gbit/s PCIe connection, only leaving ~30.5 Gbit/s of usable bandwidth), so far you can only benefit from this efficiency increase with a TB5 host / USB4 80G host, which so far outside of Apple only exists as external controllers, which are only used in giant and power hungry notebooks not in anything portable.
That the ASM2464-based stuff does not suppport supplying power and has no other USB hub functionality or a downstream port to chain another hub is a limitation of this specific controller. It was designed for USB4 NVMe enclosures, which are mostly bus powered. Thus it integrated the power negotiation stuff into the chip and has no other ports for other functionality.
There is supposedly a variant of that chip that could be used with external power negotiation, that could then also supply power from an enclosure again, but so far, nobody has implemented that (so who knows it its actually possible as advertised etc.). With the Intel controllers, power negotiation was done by a separate chip, as it is in all our Frameworks. So the manufacturer has easy control over this part independent of what the TB/USB4 controllers do. And all Intel controllers with PCIe ports always have USB3, DP and TB/USB4 ports for downstream as well.