I've never dealt with them, but with a single exception every RMA I've done has been via basic ground shipping with average inspection times of about a week in cases where I didn't pay extra for cross shipping.Pretty sure NZXT paid for the cheapest return service from UPS. I shipped it back on 5/14. They received it yesterday 5/23. Now in waiting cycle for RMA engineer to validate the AIO. Nothing has been shipped back to me.
I guess it hasn't been too long, but just annoyed at the whole process. Seems like it could have been faster. Running the system with a low profile Noctua cooler I had on time.
The single exception was with NEC. 12 years ago I bought a refurbished 30" 2560x1600 monitor from their pro grade lineup (~$2500 new, $1100 factory refurb). It had a fault stand that required me to prop the display up (I used a DVD case) to keep it from tilting down a few degrees. They sent a replacement monitor (I asked and their inventory system wouldn't let them just send a stand) from CA to PA via 2nd day air; and then used basic fedex ground to take the original one back. That turned into a pain point because the Fedex service they used didn't do residential pickup and the massively oversized box didn't fit into my car. I ended up having to borrow a larger vehicle (can't remember if it was a truck or minivan, the person I'd've borrowed from had both at the time) to transport it to the nearest depot.