Hello,
I purchased a Quantum 2 a few months before it was discontinued and this is to share my experience.

My choice of interface was based on the lower latency Thunderbolt can provide compared to USB, which was important to me for real-time performance and recording. So, I also purchased a Thunderbolt adapter for my computer's Asus X299 Sage motherboard, configured the relevant options in the UEFI BIOS and booted up Windows 10.

It took me several weeks to get the TB adapter and its drivers to work "properly". I had tested Quantum 2 with an old (2013) Macbook Pro and it worked fine there, but I needed the CPU and RAM of my Windows Desktop PC, so I had to insist. On the PC, the Quantum 2 would either not blink its Thunderbolt LED, suggesting no signal from the TB card, or it would blink but not connect.

I think I accidentally made the TB add-on card work and, once the Quantum 2 was connected, I compared its latency with my older Steinberg UR (USB 2) interface. I must say that I was not impressed at all; reducing the buffer size resulted in audio glitches (on a 10 cores 20 threads Intel i9 CPU, hardly utilised during my tests), while increasing the buffer size resulted into solid sound quality but very similar latency with the USB interface. Nevertheless, I kept using the Quantum 2.

In the year that followed, sometimes the Quantum 2 was not recognised, due to the Thunderbolt card not sending signal. A reboot would often solve this. Other times, I couldn't fix it and the system fixed itself a few days later, without me having made any system changes. Very odd, unpredictable behaviour.

I thought the TB card might be defective, so I replaced it. The new card worked well for a few months, until it also suffered from the same issues.

After a year and a half, I have lost my patience with TB. I realise it's not the Quantum 2's fault, but I can't replace the computer only because TB doesn't work correctly and, even while it worked, TB did not deliver significantly lower latencies than USB, to justify the pain of working with an unpredictable system. It looks like Asus+Intel+Microsoft should work more closely together and improve Thunderbolt's reliability on the PC platform.

I've retrieved from the closet the Steinberg USB interface and removed the TB card from my PC. That was it with me and Thunderbolt ad-on cards. Goodbye!