I live in Hawaii and bubble tea is everywhere. They have a ton of sugar.
There are different types. Many places make a slushy with whole milk, ice, and whatever flavor you chose in powdered form. Then, there are the bubbles at the bottom which are large tapioca pearls (which are 100% starch aka sugar, plus sometimes they are prepared with brown sugar also). Other places make more of a smoothie type drink with fruit, ice, and either a milk or juice base, plus those bubbles at the bottom. The bubbles alone can be several hundred calories, depending on how big the drink is and how' many they put in your drink. Then then drink itself is several hundred calories. You're looking at anywhere from 400-600 calories for one of the smaller drinks.
It is a lot for one indulgence but they are indeed good. It's up to you where you spend your calories so you have to decide if it is worth it. If you want a lower cal bubble drink, maybe dry a simple Thai tea with bubbles? Those are essentially prepared black tea with milk and sugar and bubbles. You could probably save a lot of calories with one of those instead.