I've been shooting a pair of YS-D1's since late 2015. A couple of hundred dives. Last year one of them failed, and I bought a pair of used YS-D1's as a replacement + spare. That's all I can say about reliability issues. When the YS-D2 was announced, the one new feature that I really like is the battery separation in the battery compartment. Makes it easier to swap batteries on a moving boat without having them go in crooked and get jammed up. Not a lot of differences overall, though the new controls look handy enough.
The YS-D1's are very powerful as underwater strobes go, but they are never going to light as much scene as a wide angle FX lens can take in. But it will light a foreground object nicely. Other strobes are going to have less power, save perhaps the new Retras, and they won't have enough power either.
Also, if you want to light as wide an area as possible, you'll have to use the 100mm or 120mm diffusers that come with the strobes, at cost of some strobe power.
The YS-D1's will work in TTL mode if you have the right kind of trigger, or pop-up flash.