what ill effects would be caused by using the "passthrough" output of aMirabox HDMI capture device?

In this post  JohnWayne111 listed the hardware he successfully used to implement an external frame-grabber for Hyperion/Ambilight. Its an Ezco splitter and a Mirabox capture device.

Obviously the correct setup is to split the signal from LE, with one branch going to the TV, and one going to the capture card.

But the Mirabox has an HDMI output that claims to be a seamless "passthrough" with no lag upto 4k@30hz.

Is Mirabox exaggerating their claims? OR can I skip buying a splitter and go from LE -> Mirabox -> TV?

Quote from JohnWayne111

--> Mirabox OUT --> PI 4 USB 3.0 IN

When you say "Mirabox OUT", you mean the USB output, but I want to utilize the *HDMI* output (aka "passthrough") of the Mirabox.

The RPI4 is going to be both the HDMI signal source, and is also going to have Hyperion running to snag the frames via the USB grabber and then that same RPI4 will control the LEDS. It seems bass-ackwards, but Hyperion can't frame grab directly from the V4L2 pipeline used by LE 11.

My Mirabox arrived, and I set it up as shown in my diagram above. Its a partial success.

The video signal passes through unmolested to the TV, and the Hyperion installation is able to simultaneously do screen grabs.

But, the Mirabox blocks CEC signals from the TV getting back to the RPi4, so for this particular installation this won't fly.

I guess I can try throwing some more money at this problem and order a splitter, maybe the splitter will have better luck passing CEC than the Mirabox did?

Has anybody successfully seen CEC work through an HDMI splitter?