Well constructed timber doors have and always will continue to find a place in the market. A lot of the problems associated with them can in part be blamed with the timbers used in their construction in the last 20/30 years. Mass produced rubberwood, numerous other types, in the main, what ever is cheapest in the market at the time for the buyers.
Poor construction methods using dowel construction, also plays a part in the failing of a lot of doors fitted in the past years. This continues to be used in these mass produced joinery products, reason being, it suits machine production better than that of traditional made mortise and tenon construction, and the use of engineered timber means the selection of the timber itself is less important. Take a trip to your local Howdens and look at their premium product doors using tenoned construction and then look at how it is wedged. A sight for sore eyes a lot of the time. The point always comes back to money however, todays methods are driven by everyones desire for everything to be cheap, and you get what you pay for, however if you are willing to pay good money for joinery it is out there, just not down at your local magnets and howdens. In truth however, it never has been there, ever.
The composite type i refereed to, were the metal/ fibre glassed skins on foam cores and others. I have found these to have come a long way in the last couple of years. But i would still not fit a dark colour in full sunlight.
Engineered timber doors whilst being sound in its concept are not suited to areas which receive a lot of traffic, by its nature they all really on modern glues and more to the point, a piece of constructional veneer to make it look pretty, one serious knock and you are stuck with it, no steaming it out or repairing in any way. To my mind keep veneers away from anything which is not disposal by its nature or should be kept to areas to which it is unlikely to be damaged.
I have made a large number of entrance doors over the years in all sorts of timbers, and i can say it is very rare i receive a call back for any issues that arise. Just a case of finding someone in your area. Tell them where it is located, facing south, etc, shielded, porch, painted finish/ natural timber. Let them tell you want you need and if you are unsure of the information they have given just post it up on here.
Hope that helps