Walnut can certainly show a good deal of variance in grain structure and color/tone. Mahogany is notoriously bad for this as well. A lot of times, when a cabinet shop purchases material, that solid wood may not be coming from the same trees or forest as the veneers are. This is where "Standard", "Select", and "Premium" grades usually come in. There needs to be a selection process prior to assembly to ensure that the grain structure or colors are going to match in the end. This slows down the process, ergo + labor, + lost opportunity cost in the shop -BUT- a more uniform product in the end.
With veneers, you may be paying extra for grain matching, book matching, or slip matching—which can get very expensive, very fast. All of these things drastically increase the amount of attention needed in manufacturing and it caters the entire assembly/milling process to whatever the end-goal is.
As for finding or making frame-and-panel doors that are entirely veneered... We can't speak to that since it's something we don't do. The only benefits I can imagine this has is driving price down, speeding up production, and allowing greater control for a uniform color/tone. All of this at the cost of a product that looks real, but... isn't.
EDIT:
I realize I never answered the last part of that question.
If a person has the mindset that when a door gets scratched or dented then it needs to get replaced then a veneer door will last just as long as a solid wood door, with the exception being water damage. More specifically, what I'm talking about here is a door's ability to remain looking presentable until it needs to be addressed. In this case, veneered doors and solid doors will last a similar amount of time.
When talking about durability, a door's ability to "hold up over time", IF you have the mindset to fix or refinish a door, is where a solid wood door will shine. Because the veneer on a veneered panel is so thin (industry standard in the US is about 0.6mm, premium veneers can get thicker), we're talkin' less than a 1/32", it becomes incredibly difficult to sand through a finish and stop at the veneer in a repeatable, time-efficient manner. With solid wood, you can work a little bit past your finish and sand into the surface of the wood and there will still be more wood... because it's 100% solid wood! Solid wood doors, if they're made from a nice species of wood will likely be worth the time and effort to refinish, thereby increasing their lifespan. A decent finish should last at least 8-10 years. A great finish will last 15+ years, depending how they're handled.
Keep in mind, completely refinishing doors isn't cheap. Many times, a good finish can be 75%-100% the total cost of the unfinished doors themselves. If cabinet doors get refinished professionally, you will get charged for the normal cost of a good quality finish, PLUS the time/labor of sanding through the old finish and making repairs. All of that will cost maybe 75% the total value of the original doors (including their original finish) but should cost less than buying new solid wood doors. If you're a DIY-er or you "know a guy", many times you can get this done cheaper. Be warned though, good finishing requires a lot of experience, talent, and quality products.
Q
Vertical and Horizontal Grain
Q