The main disadvantage for the unlocked iPhone devices is its price compared to the locked iPhone,
for example if you get the new iPhone 12 Pro Max as unlock you will pay it for 1099$ + 50$ tax = 1149$, but if locked you will pay 45.79$/ month, and this will be the price for the base storage one, but for more storage you will need to pay more,
On the other side the advantage are a big list of points,
Unlocked iPhone devices increase the device price by 30%.
Unlocked iPhone devices gives you the ability to switch between carrier, without the need to jailbreak or use tools like Gevey SIM.
You can travel over seas with no restrictions so you can get a short-term phone plan to save a lot of budget.
Saves more data roaming budget and switch to other carriers,
So the biggest advantage of having an unlocked iPhone is flexibility .