A pressurized system can theoretically be pressurized to any degree to force the mobile phase through any given column no matter how large. So very long columns or very big columns are no obstacle because the mobile phase will not be impacted by the pressure from above other than as a motive force.
A vacuum system is limited to only those vacuum levels that do not boil the organic solvents in the column. You simply cannot turn the vacuum up beyond a certain point before boiling solvents like alcohol or hexane when in a dense volume of stationary phase material. This is the limitation because in most respects I find that vacuum type set ups seem very well suited otherwise to cannabis separations and are easier to work with as well.