At the moment external bifold doors with flush thresholds are very popular and I understand the practical benefits of having the internal floor and outside floor at the same level but this type of threshold is designed for sheltered areas or internal applications and they also have a zero weather rating which means they have no protection against wind driven rain which can penetrate the threshold and enter the house, ruining the internal flooring. To get around this problem, manufacturers use a weathered tested rebated threshold. However it does result in a small step of approximately 20mm between the two floor levels. A continuous underground channel drainage system running parallel to the threshold may also be install as a secondary measure in the prevention of rain water ingress.
Sales people will often say that your builder or carpenter will have the necessary skills to fit bifold doors and this may be true but often they are not an accredited bifold door installer, recommended by the manufacturer. Therefore the warranty on the bifold doors will be invalidated. Bifold doors tend to be heavy and although it is supported on the hinge side, the opposite side has no support when opened and over time the doors will start to drop and you will only become aware of this, when the door does not lock properly and the bottoms of the doors catch the threshold. The reason why this has happens is because the glass panels in the doors were not correctly toe and heel. The glass must be diagonally braced using plastic packers which are wedged in the gap between the door and glass. This keeps the doors square and stops the doors from dropping. Once the bifold doors are installed, familiarise yourself with the doors, i.e. How the doors normally open and close and routinely inspect the door margins (gap between door and frame), this margin should be even. Also look out for loose screws or broken screw heads and any damage to the bottom track. This will enable you to identify problems early and prevent further damage to the bifold doors.