That would be clearing the room; many buildings have more than one room though... plus, as you illustrate it would give a false sense of security because a room that was hastily cleared via flamethrower could have held a hidy-hole (or closet) that a hostile was hiding in.
The only downside is that they really are extremely dangerous to operate.
More from the incoming small-arms fire of being a bullet-magnet than any deficiency/danger of the flamethrower itself. (IIUC, the fuel is typically akin to diesel and needs more than a typical spark to ignite.)
Actually, I believed that flamethrowers would suffocate the enemy, so hiding in an area that isn’t sealed is just as deadly as being burned.