The main problem is in standards, delays and uncertainty.
At the moment, a UK company can ship to a German company with no more paperwork than it would to ship to another UK company. There are common standards, so the German company knows that its incoming goods are compliant, and it knows that there are not going to be customs delays (I've had samples shipped from China for work, and it's very difficult to estimate when they will arrive).
For clarity, standards are generally set at a European (not EU) level, and so Brexit should have a limited impact.
