Not the timber it's the paint, or the design details.When I did "O" level 50 years ago we were taught, through M&Ts where there was likely to be moisture, as they enable the joint to dry. Stopped mortises were supposed to retain moisture and suitable for furniture and internal work. I have followed that advice making the occasional replacement sash and casement. My problem has not been the joints but the poorer quality of timber, compared to the 130 year old wood I replaced, some has not lasted 30 years!
Well yes through tenon with wedges (either side, not in the tenon) very strong tried and tested joint. Rot not a problem. Stopped mortice you'd need to draw bore and still not get anything near as solid.I've got no idea about longevity re rot but through tenons allow wedging which with a bit of glue will stop the door from sagging. look at it like this it's not possible to make a tenon a perfect up down fit its always a bit slack. with blind m and ts these will have space above and below and if the glue fails the door will sag. wedges banged into through tenons( the ends not in the tenon that's for show)are solid and under many kilos of wedging action which can make the bond extremely strong.