Cost piles up faster than it feels. A $25 disposable that lasts a week is roughly $100 a month. A 30 mL bottle is also around $25, and it lasts most casual users two to three weeks. The kit you pour it into is a one-time spend.
Flavour and strength are locked in. You bought whatever was on the shelf. If 20 mg felt too strong, you can't drop to 10 mg without buying a whole new device.
Flavour fades before the battery dies. The cotton runs dry before the lithium does, and the last 50 puffs taste burnt.
Disposables become e-waste. Each one is a tiny lithium cell glued to a coil and a tank, and that doesn't go in household recycling. Drop spent devices off at any vape shop for proper handling.