I have an EL-EN15 in my D810 and so far no problems. What one needs to be aware of when looking at after market batteries appear to be cheaper but are they. Al the Nikon copies I have seen are only around the 1200 - 1500mA/hr where as the Nikon is 1700mA/hr. the cheapies will not give you the same number of exposures or the burst rate of the 1700mA/hr batteries.

Does it harm the battery to leave them in the charger? Depends on the manufacturer, some chargers have temperature sensors to stop any overheating while charging. I have seen a number of Li-on chargers for battery drills will discharge the battery if left in the charger. They discharge via the temperature sensor in the charger. As far as I can tell Nikon chargers do not have this problem.