Rain flys are usually a compromise between coverage and weight. Some provide more coverage, but at the cost of more material, so more weight. That rainfly looks like a decent attempt catch most of the rain with a minimum of material, so if that's your goal, it is likely a good option. If you need the rainwater collecting aspect of the hammock fly, it might be useful. If not, it seems over priced.

The part about it being open at the ends, yes, that's an area to watch, but it's not necessarily a problem. If you expect rain to be blowing in from the ends, you can:

• move your hammock to different trees to keep the hammock perpendicular to the wind.
• Lower your tarp as close as possible to your hammock.
• Use some kind of "drip string" on your hammock line to divert any rainwater that comes down the lines towards your hammock.

The tarp I've been recommended and is on my "to buy" list is the Superfly. It might take up more room than the one you mentioned, but it's got a lot more coverage and is considerably cheaper.

Cold weather concerns are valid. There are solutions. I'm not a big one for cold weather camping in any structure, but I've had my Hennessy into the upper 30s, I think. I used every method of temperature protection to stay comfortable at those temps, but it was largely successful. I don't how it compares to tent camping at the same temperature. The biggest thing is to get insulation that hangs outside hammock and hugs the underside, an underquilt.

There is all kinds of information to be found at https://hammockforums.net/