There are some problems that are very related to putting feet into ski boots. I will briefly go through some of the most common problems divided into three subgroups.
1. Volume related problems
In general this problem area relates to fitting issues with bunions, bones and thickness of legs. You will minimize initial volume problems through a really good customer assessment and shell fit to start with. You will then fix smaller problems through shell and liner modifications. Punching, stretching, grinding, cutting, adding material and so on.
2. Nerv and blood flow problems
This topic relates to specific sensitive areas of the foot where nerves and blood vessels easily get's a lot of pressure. One extra important area is the dorsal side of the foot (the side you see when you look down on your feet) especially under the second buckle of the boot. Here you will have nerves coming down just under the skin above some bones and if you have improper fit (too tight fit) of the boot here you might end up with a lot of problems. Some of the more usual symptoms are a numb forefoot, cold feet, burning sensations and so on.
Another area that easily can cause problems is around the ankle bones, more specific on the posterior side (as if you look at the heel from behind) of those bones where nerves, blood vessels and tendons comes down the foot.
You must let the nerves and blood vessels have enough space to function normally and not causing any discomfort.
3. Position and alignment problems
Now we are starting to talk about very important things where you could earn god like status amongst your customers if done correctly. As I see it you have two levels here. Level one are not that tricky and if you adress this correct you will help your customer to ski better with a less energy consuming style.
I once heard a well know editor for a ski mag say. "Stance alignment is not really important for average skiers or freeriders, only for racers". I mean come on. Of course it is important. When doing activities you always do better when you are in an advantageous position.
So what is alignment problems? It is basically when you are out of your ideal position when skiing which will for example cause you to have more strain on your muscles to keep yourself in dynamic balance. You will also find that it is more difficult to edge your skis quite effortless and hold a good grip on the snow. You will find it more tricky or exhausting to ski in uneven terrain or moguls because you will be out of balance a lot. It is also when you will struggle to initiate a turn or exit a turn.
It is important to be aligned the best you can, pro or not, to have the best skiing experience possible.
Level 1: It's like building a house with a solid base. You will need the correct boot to start with. Good fit and all that. Right type of flex pattern and stiffness of the boot. You will need good custom insoles made that's keeping your feet neutral and stable inside the boot and pair that with a suitable angle of the boot board and forward lean on the cuff. You can relate this level to the view of a skier in a good position in the sagittal plane (side view).
Level 2: Here you will fine tune and do smaller adjustments. This relates a lot to the ability to do side to side adjustments on the skis or edging the skis. You will seek for advantageous angles of the boot sole, proper cuff alignment and so on to keep you balanced. You can relate this level to the view of a skier in a good position in the frontal plane (front view).
Start building the base and then fine tune that base. Do not do it backwards!
There are a few more things to say about common boot related issues.
1. Arch cramping - Most likley the boot is to big and you are tensing up the foot to get it more stable.
2. Cold feet - Most likely the fit is to tight over the instep area and you have wet liners and wet socks.
3. Burning thigh muscles - Most likely there is a alignment issue in the sagittal plane and you are leaning to much back when skiing.