That is a really good question. HTC technically reserves the right to void a warranty for any phone that has the bootloader unlocked, and yet this phone comes from the manufacturer BL unlocked. In practice it seems (from reports on XDA, at least) that HTC's enforcement of their policies is inconsistent at best. I've seen reports of unlocked bootloader is ok (warranty still honored) but custom ROM is not (even if returned to stock ROM, there apparently is some mechanism to tell if a custom ROM was ever installed); some reports of modding ok since the failure was hardware related, and other stories.
I suppose you can raise the question to HTC. But what they tell you, and what actually happens in practice may end up being two different things.
As far as compatibility with AT&T's network, from my understanding the hardware is exactly the same, just a non-AT&T branded ROM and bootloader unlocked. So it should work just fine on AT&T's network.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)