Most of the time when a customer sends a gadget in for repair, under warranty or otherwise, the manufacturer will either fix the problem with the original unit, issue a refurbished unit that's also been fixed, or (if supply and/or time is low) send out a new one. None of those three options seem to be available for certain ailments of the Nexus 5X. According to an Android Police reader and several posters on the dedicated 5X subreddit, LG is issuing full refunds for owners that are suffering from a particular malfunction that causes a "boot loop" and an unusable phone.
The problem seems to be availability of parts. Remember that Nexus devices are produced in relatively low volume (at least compared to a flagship device like the G5 or V20), so specific parts like the unique case, screen housing, or phone motherboard are produced in smaller batches too. Since the Pixel phones have been released, the Nexus 5X is no longer being sold by Google or (presumably) manufactured by LG. The service email to multiple customers says that the company simply doesn't have the parts available to fix this specific problem, which would indicate that said part isn't interchangeable with a more mainstream device.
From the Reddit post:
We're sorry to hear that you've experienced a problem with your LGH790 and appreciate you giving us the opportunity to provide a warranty repair. We have received your device at our repair facility but currently a part to complete the repair is out of stock and is no longer available.
To rectify this issue, we would like to offer you a refund for the full amount of your device. That amount will be determined by your sales receipt. Your refund can take approximately 4 weeks to receive due to the holiday.
Whatever the reason, reactions from the handful of owners who have been sent the email is positive - apparently LG is offering a full refund of the original retail price after going through the company's lengthy verification and shipping process. Remember that the Nexus 5X (along with the 6P made by Huawei) was released in October of last year, so many owners will still be within the standard one year warranty period. If you're experiencing boot loops with your Nexus 5X, it might be a good idea to start the service process sooner rather than later. Note that the emails are coming from LG's customer service, not the Google Store.