Freelancers (i am one myself) often have a fear of binding themselves to a single company for a longer period of time. This can be good or bad. When i got interviewed by a top european startup their HR thinking was like "Freelancers are more risky, because they might get bored quite fast and leave". Thats true to some extent, on the other hand i personally like the idea to really go deep on a project for 2-3 years instead of jumping from project to project.
In your specific case i wonder:
- Would this be a project that you would want to work on for next couple of years even if it doesn't take off immediately ?
- why did the other programmer leave if the company seems to be so well off ?
- how good is the salary really ? If they had money, wouldn't they find good people in Hamburg quite easily ?