While this is true in a way there are certain things that you can do to eliminate much of this. Using chemicals similar to a swimming pool or hot tub you can control the PH balance of the water in your pond. Keeping the PH balanced will eliminate the breeding of mosquitoes. As far as the moss goes if you keep the water circulating with a small pump this will cause you very few problems if any. You can also add another chemical from swimming pools such as an algae-guard. As far as frogs go, the only way to keep out wildlife is to clorinate the water. You can basically treat the pond just like a swimming pool to eliminate many problems that you could anticipate. Chlorine will also keep the water in your pond clear and prevent the discoloration and cloudy look. Chemicals to begin to cost money over time but for a small garden pond it should not be bad at all.
I am looking to invest in a garden pond for my front yard possibly this winter so that I can have everything ready for next summer. The ones I am looking at are not portable but are rather cheap to invest in. It basically consists of digging a small hole in whatever shape you choose, loading it up with gravel, having a piece of special rubber liner cut to shape and fit for depth, and placing numbers of large step stone type rocks to accent the edges. It will cost me a bit more to install the plumbing for the pump system I intend on installing as well as the waterfall fountain but the basic pond kit starts at around 400$ US.