What downsides for replacing WiFi with an unlimited mobile hotspot?

Lets say I live alone with somewhat infrequent guests. Is there any real reason to pay for two internet services, when my phone could do all the heavy lifting?

It's not the WiFi you're replacing, it's the wired connection behind it, right?

Cellular interference is subject to so many things from the buildings around you, population density and even the weather. Imagine the volume of space between you and your mobile tower and how many things might disrupt your signal from reaching the tower. It's like the difference between riding a bicycle on a city road versus a dedicated bicycle lane.

Phone data service is not really unlimited- they will throttle after a while.

Also, the data latency and packet size are much worse over mobile networks.