The Big Roboport lack 1 range in width versus height :

I can put 193 belts beetween 2 roboports vertically, while I can only put 192 belts horizontally

If i use normal roboports with max range beetween them, then i can replace a vertical line of 5 normal roboports by 2 huge roboports, placing them at the exact same place : no range deperdition
The same doesn't work when considering horizontal layout : the 1 unit space/distance lost mean you have to change the whole layout, it isnt square anymore and it is inconvenient in a megabase setup