Where it earns its place
Low volumes, seasonal trade, a second point of sale for the odd queue, or a run of jobs that never justified a dedicated device. It is also the fastest way to stop turning away a card payment because the terminal is back at the office.
Since the surcharge ban removed the option of passing card costs on, cutting hardware is one of the few levers left that does not involve renegotiating anything. This is the main one.
Where it does not suit
High volume at a fixed counter. A phone is slower to hand back and forth than a terminal on a stand, and at pace that difference shows up as queue.
It also ties your takings to a device that rings, runs out of battery and goes home in someone's pocket. If several staff take payments, a shared terminal is usually the saner arrangement.
What to check
Which phones are supported, whether it works with the software you already use, and what the rate is — tap to pay is not automatically cheaper per transaction just because there is no rental.
That is the real comparison: no rental against a possibly higher percentage. Which wins depends entirely on your volume, and it is arithmetic rather than opinion.