What is there to smuggle?
The answer is bold and bright: EVERYTHING!
It depening on the situation if the bennefits are enough for trafficing the goods over de border. It's just a question of Supply & demand.
Fuel and alcohol are in Belgium and Luxemburg way much cheaper then in the Netherlands.
Asperinin are in the Netherlands without perscription and for a much cheaper price available then in Belgium (The price in the Netherlands is 1-2 euro, in Belgium 6-7 euro)
The distance to the border en the differance in price that determan of the benefits are paying off.
Legal limitations determin what can be brought across the border. (customrules NL)
All that is brought in more can be considered as smuggeling.
Consider that as a violation.
What is/was smuggled mostly?