Anything that has water in it is susceptible to corrosion. The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward. I see your point about the dropping prices making PV production lower I still think that direct heating i the most economical and easiest way. Without hard numbers from a a properly conducted study, I am just going by experience. If you have a something with hard numbers I am very interested.
I can't give you "hard numbers from a properly conducted study". I can give a quick analysis of what it looks like for me. I installed a
Nyle Geyser heat pump water heater and the measured power it uses to keep our home (2 adults and 2 small kids) in hot water is 2 to 3 kWh per day with normal use. Obviously the amount of PV needed to provide this will depend on the solar insolation but of course so would the area of direct solar water heating panel needed.
For my location, 1,000 watts of PV can produce this on most days. Since i already have a PV system, the cost of adding the additional PV is about $1/watt (I paid $0.72/watt for my panels). The Nyle Geyser cost me about $700 IIRC. So about $1700 total (I did my own install). BTW Permit fee here in WA state for this is less than $100 and in my case I was already expanding my system so no extra cost.
To get the equivalent with direct solar hot water heating, i would have to install a 40 sq ft closed loop system (it freezes here) for about $4000 (assuming i could install it myself which is questionable!).
Also since i have grid tied PV system (with battery back up), i can sell any excess PV back to the grid on sunny days.
I realize my situatuon is different from someone who doesn't already have a PV system in place and i'm not arguing that using PV to heat water always makes more sense than direct solar water heating. It's just that the previously held common wisdom that heating water with PV never makes sense is no longer true given the low cost of PV now.