MIX 3 has global 4G bands meaning any of the US carriers can support it. Most fans of the brand prefer T-Mobile though because less shenanigans compared to Verizon and AT&T.
Samsung failed again and again pulled marketing fluff. The slider currently is the best and basically only way to get true bezel-less without the compromises. There is something to be said about mechanized selfie apparatuses but mechanizing means that any of the gears break and the phone is done. Meanwhile the slider will only ever loosen or get stuck and your phone will be fine regardless.
On the topic of what Samsung claimed to be doing Chinese OEMs were already at it half a year before Samsung realized they could market the idea as their own and promote their brand as being innovative without actually doing anything. The brand new S10 is both a copy of Huawei's hole punch design and a shameless fall into the notched smartphone design that its fans spent the last couple years after iPhone X came out bashing. Now these are the same people praising the notch on the S10 and Galaxy Fold.