Milk tea on its own is not a big problem. At 110 calories a cup it is less than a can of Coke (140) and it comes with about 3g of protein from the evaporated milk. The problem is frequency. A lot of office workers in Central drink two a day, one at breakfast and one at 3:15pm with the afternoon tea set. That is 220 calories a day, over 1,500 a week, from a drink they do not count.
The evaporated milk is the calorie driver, not the tea. Black tea is about 2 calories per cup. A cha chaan teng adds around 50 to 60ml of evaporated milk, which is 70 to 80 calories and nearly all the fat. Then 7 to 8g of sugar adds another 30. If you order cha jau (茶走), where condensed milk replaces both the evaporated milk and the sugar, the cup goes up to around 170 calories because condensed milk is roughly 50 calories a tablespoon.
Iced is a bit higher than hot because the glass is bigger and needs more sugar to taste right over the ice. CFS puts iced milk tea at 130 calories at regular sweetness with about 22g of sugar, which is more than half of the daily 40g limit the WHO suggests. Order it siu tim (少甜, less sweet) and it drops to 100 calories and 15g sugar.