It doesn't seem like it should be complicated, but it is. Everyone has a different idea of what a "Get Home" bag vs. a "Bug Out" bag vs. and INCH bag. So, that's one problem. This is related to comfort, durability, capacity and cost.
The question of weight/capacity. Some packs are more comfortable when carrying weight greater than say 25-35 pounds. Good comfortable packs that can carry WEIGHT are expensive and need to be more custom sized. A lot of these have frames internal or external. Consider capacity. You don't have to fill the pack up either, so a general BOB in the 60 liter or larger size can carry most of the stuff needed for "get out of town" headed for a stocked location. But if you use restraint you can get a little larger pack and not fill it. Some packs are better for handling varying loads than others.
I'm currently interested in Mystery Ranch hunting packs that use a frame that allows you to vary the load. Makes it sort of modular. My thinking is that I keep a BOB of standard survival stuff, but the frame allows me to strap on a duffel with extra food, cold weather gear, ammo or what ever for any changes in mission. Keeps me from buying multiple packs.
Here is a Mystery ranch with the pack extended for load behind it. MR makes several of these. There are other brands that make something similar as well.
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Standard backpacking packs can be made to be light weight rather than durable. So, there is a trade off because standard backpacking packs can be cheaper, but the light weight material leads to the possibility of failure at the wrong time. If you don't plan to use the pack except for emergency this can be a good solution.
Speaking of cost; Another reason I like the modular approach is that it is expensive to keep a BOB that is never used except to practice. Having a frame pack that is made for adding load will give me some flexibility. I can keep the core survival stuff in the main bag for BO, camping use, hiking, etc., but add a larger duffel strap on for longer term.
I've been using a London Bridge Trading company 3 Day Assault Pack for my BOB. It is VERY tough, but it is just doesn't have the capacity. Even with molle attachments it carries what I consider only the essentials for an emergency pack, with little food. That's why I'm about to take the leap to MR.