DS,
Like a lot of companies on the forum I used to outsource my doors and drawer boxes.
The drawer boxes were typically maple lumber dovetail. The doors were usually cope & stick maple.
As you know, there are a half dozen possible places for drawer boxes to fail.
They can be the wrong size, out of square, too much putty in the pins, too much hot glue on the bottom. Occasionally the groove on the bottom was produced with a cutter head that should have been replaced a couple of jobs before mine. Not all of these defects would be manifest in every job but it happened often enough that I would need to have something replaced.
The problem with the doors tended to be how flat they were. Most of them were acceptable but at least one or two weren't. The door company catalogs would usually include a caveat that stated a warp or twist of 3/16 inch is not considered a defect. This grade of door is fine for overlay construction and almost feasible with concealed hinges but most of my work is fully mortised butt hinges.
When I first started to build with smaller batches I noticed the work would start puddling up in the shop. This usually had something to do with a door or drawer box that wasn't in the building.
During this time I had a retail showroom at the front of my shop. One of the displays had three different types of drawer boxes. One of them was an outsourced dovetail drawerbox, one of them was a melamine drawerbox and one was an appleply drawer box with pocket screw construction.
All of these drawers looked the same when the drawers were closed. The only difference was the cost structure. My showroom sold to a random mix of customer. I sold $8,000 kitchens and $80,000 kitchens. We sold projects in excess of $100K so often that I didn't even bother to mention it to my wife. It was just what we did.
Just about every single customer I ever had simply did not care. Since we could cause a pocket screw drawer box to happen in about 20 minutes I just moved away from the dovetail.
I did, however, have a few customers that insisted on dovetail construction. With very rare exception these individuals also had other PITA characteristics. Dovetail drawers are now Step 1 in the rorschach test for us. We won't necessarily rule you out when you ask for them but we be a little more vigilant while we are interviewing you.
The doors are a different story. We have to have a really flat door. It has to start out flat and stay flat. For this reason we developed a process to build a flat door. It has a few extra steps but they are simple steps. We can teach a greenhorn woodworker to become pretty competent at making a flat door in about one week. Our system always produces a flat door.
Lumber quality is key to flatness. The problem is that only a portion of the tree is conducive to flat stiles and you never know whether or not this is the lumber that's going to show up. Once the lumber gets into the building you have to be able to read the board and you have to be able to coax straightness out of the board.
The process we use makes all those considerations moot. We bring our lumber in already ripped to 30x30 mm squares. The lumber sits at our distributor's warehouse for a couple of weeks before we see it. When it comes in we laminate it into three piece staves. Every stile and every rail in our cabinet doors is built out of three pieces of lumber.
This is an extra step but it is an entry level step. We built a pneumatic press to glue these pieces of lumber together.
This process generates intrinsically flat stock. The lumber will still expand and contract with changes in humidity but there is not an inherent difference in expansion and contraction from one side of the board to the other. Breaking the board into three pieces decreases the likelihood of warping or twisting later in the life cycle.
To better understand this take a look at the growth rings in a tree stump. Some years the tree grows really fast and the growth rings are plump. Sometimes there is a paucity of sunshine and/or rain and the growth rings are small. Your lumber stave represents a range of prosperity. Sometimes there isn't much difference in the diameter of the straws. Sometimes there is a lot. The arrangement of growth rings also is influential. If the arrangement is concentric the board can cup or swell differentially.
This is definitely way out in the weeds stuff but equally as important as it is tedious. We need a flat door. We get a flat door. With fully mortised butt hinges you can't really do it otherwise.
I am thinking, however, of launching on overlay style kitchen business. I want to do some experiments with price transparency. This business might rely more on outsourced doors. (We are planning to outsource boxes to you).