I have two post-recall YS-D1s on an OM-D using fiber optic sync and TTL. They are outstanding as far as I am concerned. It is very easy to adjust lightng ratios even in TTL mode, they are very powerful, and come with all the mounts and diffusers you need standard. I have a couple thousand shots and about 20 dives with them, as deep as 120fsw, and so far they have worked perfectly.
So, the initial problems seem to be resolved. As others have said, long-term reliability is still unknown until several years pass. The Inon Z240 is well regarded as the champion of long-term reliability so far. I took the chance on the YS-D1s because S&S stands behind them, and I saved almost $450 over the Z-240s.
I don't know about floods. They use the same time-tested battery compartment door as on the YS models for many years, so I would not expect flooding issues there. However, you might search for threads on that.
So, for the time I have had them, a very high recommendation. As for capability, they are head and shoulders above the S2000 in terms of power and exposure flexibility. The S2000 is for a different market, very compact, simple strobes for ultralight travel. The YS-D1 is more of a fully professional strobe like the Z440.