It is illegal. Again, the main disadvantage from which all the others flow is that illegal work is illegal in Germany. Germans are law-abiding and often turn in illegal workers themselves to the police. The police also carry out regular raids to detect illegal migrants. "Illegals" face deportation from the country. The minimum period of deportation is five years and includes a ban on entry not only to Germany but also to the whole of the EU.
Problems may start at the border. Non-EU nationals travelling to Germany are carefully checked on arrival at the airport. They may check your luggage, ask questions about the purpose of your trip, check your documents, require proof of hotel reservation, sufficient funds in your accounts for your stay and a ticket with a return flight date. If they are not satisfied with any of the above, you are sent back home. If you are suspected of entering Germany illegally for the purpose of employment, you will be placed in a detention centre and deported.