Detachable water tanks should be easily removed, not tricky to extract and replace. They should be durable, made of strong stuff that can stand wear and tear in the kitchen or office. And they should have capacity, be able to hold enough water to make a decent number of coffees before needing a refill.
Take the Cremiere TCM 106A as an example. It comes with a durable, easily removable water tank that takes 1.5 litres of water. This is enough to make about 30 cups of espresso. That’s a lot of coffee.