It also depends a lot on the trees You use; there have been accidents in European zoos f.e. with colobus monkeys escaping and crashing down from the height, as some European tree branches didn't seem to be elastic enough. Another subject is possible plant poisoning.
There are different variations of gibbon exhibits I have seen so far: the one in the Wilhelma f.e. is a rather extreme one(a tall ashlar shaped exhibit), and there are plenty of newer "bamboo variants" in many European zoos (f.e. Vienna). The aspect of bamboo is btw. an interesting one: according to a vet working in an Vietnamese primate center, the gibbons there all seemed to prefer the bamboo and not the ropes. She also remarked that a lot of the gibbon zoo exhibits are not long enough to allow the gibbons their brachiation at full speed.
Personally, I do consider the naturalistic tree version on an island the "best-looking" one; but beside the danger of drowning, the disadvantage in seasonal climate though is that the moats around these "islands" ice up in the cold-which means that You have constantly to destroy the ice or, what is more common, keep the gibbons inside all the time.
In a German book about primates, the author remarked something interesting: in old Chinese poems & pictures, gibbons are depicted as "living in the snow". While nowadays zoo are discouraged from keeping their gibbons outside in the winter, it seems as there might have been populations of Gibbons (species? hoolocks?) in China in former times inhabitating colder climates. Too bad they seem to be gone...