Are you aware if there are any issues that affect either the 850 EVO or the 860 EVO range of SSD's?
No none.
The newer drives have some significant changes vs the 840, the 840 essentially taught samsung some lessons, you learn from mistakes more than success.
The 860 EVO has a much better error correction algorithm, both 850 and 860 EVO have 3D NAND meaning larger manufacturing process. The larger process is what affects reliability and endurance. They also have pseudo SLC support, pseudo SLC is more endurant than MLC.