I've bought a couple of Firecrest filters recently and have been happy with them. Mine were a 10 stop and a 5 stop so I could stack them.
Generally color casts become an issue at 10 stops and higher. The main reason is that until recently, most filters did not block IR spectrum. They only blocked visible light. Your camera sensor has about 15 stops of IR blocking on the sensor. So if you are not blocking IR, at the 10 stop point you have underexposed IR light that starts to be noticeable. The 15 and 16 stop filters were much worse. The IR light has a magenta cast, but with white balance you could turn it into different colors - still a color cast and all wrong.
The Firecrest filters and several other recent releases have started using films that block IR spectrum as well as visible light. I've tested them on my IR converted cameras and confirmed that they block IR light in the same manner as visible spectrum.
I have one tip that may help in using strong ND filters. Be sure to cover the viewfinder during your exposure. Light can "leak" in through the viewfinder during long exposures. You can get light leaks from other areas such as the focus distance scale and even the mount, but those are less common. If possible, try to avoid having direct sun hitting your camera and lens if you find problems with light leaks. Light leaks are either light areas or magenta areas that show up in your image. They have a variety of shapes and sizes depending on the source of the leak.