Before going fully 4K, I only bought discs that were a clear improvement over their blu-ray counterparts. But all things being equal, a blu-ray will tend to look better than a UHD disc (assuming they're from the same master) on a 1080p set.
I think it depends on the player. I bought a Panasonic which lets you tweak the picture settings and never had any issues until I eventually picked up a 4K display.