As you approach the end of the new section, the speed limit drops to 55 and then to 40. It ends in a 90-degree turn onto the Mullens Connector, which is signed as "To WV 54" and "To WV 121" rather than WV 121 proper. WVDOH did not build any stub of the mainlane past the interchange, so any future construction to extend the road will impact the mainline. The road basically ends in the side of the mountain, so a lot of earthwork is going to be required for an extension.
Heading into Mullens, the new connector is two undivided lanes (no climbing lane) and is quite twisty. It is signed for a mile-long 11% grade. I hate to see what this will be like in the winter. I suspect a lot of people will stick to WV 54 during bad weather.
The new road improves connectivity to Mullens, but I'm not sure how useful it is beyond that at this point. WV 97 is/was the preferred route from Pineville and beyond to Beckley because of its better alignment over WV 16 west/south of Mullens. I don't know that the new route really changes the arithmetic. WV 54 is already a relatively modern road between Mullens and Beckley. I think WVDOH would have been better off focusing its efforts west of Mullens or perhaps south/west of Pineville, where the new road would have represented more of an improvement over the existing network.
I'd like to think that the current connector at Mullens will eventually be removed, or at least be routinely closed in winter.
Just doing the math in my head, the connector road to Mullens must climb well over 500 feet. I suppose WV 54 climbs a bit as it heads north along the river from the connector, and the future expressway could descend another 132 feet if it dives into a 6% downgrade for the 2200 feet between its current end and where it would cross WV 54. Still, that's a vertical difference of about 400 feet at best. That suggests to me that there's not really a place for a reasonably benign connection from the expressway to the legacy road network anywhere near Mullens. Hopefully I'm wrong about that.
I guess I should get off my butt and research the situation, but for now I'll just say: building a new four-lane mountain expressway that's accessible only via a mile-long road on an 11% grade in a region with icy and snowy winters was a very bad idea. The recent announcement that the next phase of the expressway will be noncontiguous rather than extending the expressway to a more suitable access point seems to compound the mistake. What were they thinking when they decided to end the latest phase where they did? What are they thinking now? 
The Mullens connector is it and all that is ever going to connect to WV 54 at this location. It's not going to be closed in the winter, unless temporarily if it's a solid sheet of ice, because that's the only way to get to the road. Other mountain crossings on existing roads have comparable grades in this part of the state, although most have better sunlight exposure to promote melting snow and ice than this one does.
IMHO, building this section of the Coalfields Expressway was pointless. It duplicates existing good corridors (WV 54) and doesn't provide access to anything between Slab Fork and Mullens. After the initial section, which serves as a Sophia/Lester bypass, they should have swung over to WV 54 or WV 16 and dualized/rebuilt one of those corridors. It would have provided better connectivity, required a lot less earthwork, and not resulted in a new terrain alignment to maintain during winter.
There are some fairly steep connector roads in other places in West Virginia. The Scherr connector from US 48 to WV 93 comes to mind. Both sides of the WV 65 mountain connecting the new segment of US 52 in Mingo County are also examples.
WV 65 is pretty extreme to the south, but not bad headed north toward old US 52. Also, WV 65 was relocated at considerable expense to create that intersection. I must surmise that, even with a mile of 10% grade, new 65 must be better than old 65, or they wouldn't have bothered with the full relocation. Tangential to the topic, but there are still Streetviews on old 65! You can pan around to see the WV 65 relocation under construction.
Other than having climbing lanes, the new route isn't appreciably better. The horizontal alignment is improved but it's much steeper. The old road ran through a gap that was several hundred feet lower. The road was relocated because, if the King Coal Highway is ever extended, Low Gap where the old road ran will be filled in.