1. It’s too hard to get the eggs from the nest box.
When you design your coop, keep in mind that it has to be simple for you to gather up the eggs quickly. Whatever you do, don’t make it too difficult to get to the eggs - crawling on your hands and knees through chicken droppings is something you will get fed up with pretty quickly even if there is something nice at the end of it. Some coops are built so you don't have to go into them at all to collect the eggs. They will often just have some kind of door or flap to access the nests from outside like the one shown below. If you do go for a design with an outside nestbox access be sure to make sure the catch is secure (some predators such as raccoons can work out how to lift easy catches and have the 'nimble fingers' to do so).
2. It’s impossible to move, what was intended to be, a mobile coop.
This happens quite a lot. Large mobile coops that perhaps started off a little bit too heavy to be easily moveable and then over time seem to have slowly and painfully grown roots. Perhaps designed so that the owner could move it by hooking up a truck or tractor whenever they want.
Unfortunately, over time, ‘whenever’ can easily become never.
And so the coop sits in one spot.
Instead, if you want to move your coop around regularly it is probably more likely you will do so if you plan for it to be moveable by yourself or with 1 other person. Otherwise, perhaps design it as a stationary coop.
If the coop is moveable the chickens can have access to fresh earth every so often which can be good for sanitation and a 'wire skirt' can help prevent diggers. One benefit of a stationary coop is that more attention can be paid to predator proofing such as digging down wire or concrete etc which is obviously not really possible with a mobile coop (at least not if you want it to be easily moveable).
The above A-frame coop is in theory moveable by 1-2 people if built with light-weight wood but it could easily grow roots and become a more permanent fixture (either accidentally or intentionally).
3. It’s too difficult to clean the coop often enough.
Similar to the issue of making egg collection easy, you also do not want to be stooping a lot or crawling about through chicken muck to try to clean the coop out. Making sure your coop gives you access for cleaning out is an important consideration - good ventilation and regular cleanouts can help prevent illnesses and amonia and dust building up in the coop. Some people do use a deep litter method but even that still needs managing and so easy access is always a good thing.