It would probably be ok for a young and juvenile snake, but might be too small for an adult snake.
Young boa constrictors are at least semi-arboreal and many people report that their snakes like to climb big, sturdy wood decorations. Older and bigger ones avoid climbing on wood, but still make use of shelves if these are made attractive enough with heating and a hide.
However, the length of a snake enclosure should be at least the entire length of the snake, so that it can stretch out completely and move freely (without running in circles and literally biting it's own tail). If the only way for a snake to completely straighten out is to do it vertically, the snake won't do it.
If you set up a 120 x 50 x 50 cm vivarium horizontally and include 1 shelf at 25 cm height, you create a very big living area for a boa constrictor. If you put it up vertically and include many shelves, your snake might not use all of them. You also need to cut away big enough openings from each shelf to ensure proper ventilation. It's the same outer size of vivarium, but the actual living area is smaller and cut into many small sections without offering enough space to stretch and slither around.