A chicken wing is the fattiest part of the bird because of the skin to meat ratio. Hong Kong dietitians put three plain wings (about 100g of edible meat and skin) at around 220 calories, and suggest three per meal as a sensible cap. Swiss wings are braised, not fried, so no oil is added in the cooking. The extra over plain wings is the sauce, which is dark soy and rock sugar reduced until it clings, and that is where the 12g of sugar and about 55 calories come from.
Compared with the other things on the same menu this is not bad. Three Swiss wings at 310 calories beat a baked pork chop rice (1,300) by a mile and come in under a plate of sweet and sour pork (800 with rice). The problem is what they come with. The Tai Ping Koon afternoon tea set is two wings with rice or noodles for HK$74, and the rice or fried noodles under them is another 300 to 450. Eaten as a side dish with a shared plate of greens they are a decent protein option.
The sauce is reused all day at a lot of places, which means it gets saltier and sweeter each hour as it reduces. Three wings can carry 900mg or more of sodium. Eating the skin is the difference between 310 and about 230 for three, since the skin holds most of the 19g of fat. Whether that is worth it is your call; the skin is the reason people order the dish.