When you open up the floor you create the possibility of water coming in through the hole at times of high rainfall.
French drains are best made outside, where you can leave the trench/ditch open while you find out how much rain it can cope with. The problem with a French drain is that while it is open you can see where the water is coming from, where it is going and if the slope and down hill, or pumped sump can cope, but once it is filled in, you loose perhaps two thirds of it capacity and that may cause a problem with back up.
An open ditch on the outside, will mean that the only moisture in the wall is rising up from the foundation, once you fill the ditch flooding may cause the water to seep through the walls.
Water will rise four feet up a wall from the highest point in contact with the soil.