I think it is better to refer to the GARMIN website as this thread may mislead people as it will never be exhaustive and accurate as much as the GARMIN SITE which provides such simple information as long as one as tried to look for it. An example based on my device : The list only lists the Montana 850 and 850t. If they supports NTU, then... the 680 / 680t will both obviously also support it. I've just tried to look for a Turkey NTU map from the French Garmin website ; on the map page, there is a tab that list all of the compatible devices... the 680t is NTU compatible but not in your list. Enough to stick with information provided on the Garmin site and avoid to bother with approximation found here and there on forums.
Regarding NTU now, even if the device supports UNICODE (NTU) maps ; it all depends on how one plans to use the map and I think it is important to explain this.
Let's imagine that you don't speak Russian and you plan a trip to Russia. With the help of a NTU map, you will be able to share the maps which show cyrillic names with russians who don't speak English or your native language (like they will be able to do a search in their own non latin language to help you to find a place). But if you don't read nor speak Russian/Greek/Japanese/Chinese and so on however, on your own and without the help or a native, it would be obviously better for you to read Moscow instead of Москва or Tokyo instead of 東京 on a map (not always... that's true but it is important to consider why it's better to have a NT or NTU map).
In most cases, unicode means more bits, so more data, so bigger files (computer sciences wise and generally speaking of course and not considering utf-8 and thinking exclusively Garmin map images as I haven't even checked if a NTU *.img is bigger than the *.NT version actually).
What I know by experience. For example, I've been several times to Japan, can read around 700 kanjis, most commons signs and expressions and having used latin encoded map of Japan have been not only very fine so far, but much more convenient for my brain and comfort as a temporary traveler who needed to follow road / trail directions as fast (and conveniently) as possible. Having rented japanese cars with onboard japanese GPS (in Japanese only, I tend to prefer to use Latin/English Japanese maps (I don't talk necessarily about OSM or free ones... I have olds but very English Japanese map for Garmin both topo and road oriented).
At last but not least, many Garmin devices and their OS are close to stone age in terms ergonomy and menu and screen quality. The only added value between my Montana 680 and a bullet proof and much more versatile IP68 android device (with 4G and WIFI and better camera and audio and so on) more than two times cheaper is the GPS sensitivity (maybe). The Gamin Montana (gloves friendly... that's true and this much appreciated in the mountains or as a motorcyclist) will be my last handheld GPS devices I guess... so NTU to NT you know... ;-)
Nowadays, free and collaborative maps are very good and smarphone based GPS solution are much more powerful (and even more robust and even lighter) both for road warrior, sailors or experienced hikers.