This back road from Llandeilo to Carmarthen makes a fine alternative to the A40. The first part is indeed a drag strip, with visibility for miles. Watch out for a couple of entrances that traffic might turn into, and one semi-blind crossroads. Not a serious problem - unless you are doing 140+ and the tractor that just turned out in front of you because you were a spec on the horizon is only doing 10...
Then the road gets narrower and more interesting (read technical). The surface is variable, some bits worn out and gripless and others freshly surfaced (watch for damp patches under the trees after rain, and also gravel wash across the road), and there are a few blind crests that conceal junctions, but the hazards are fairly obvious if you keep your wits about you. I saw a copper down here once, but I expect if you find one it will be lurking in one of the short lower speed limit sections of the road.
Much better than the busy A40 or A48