Being a startup, it's understandable for this salary, but the senior software engineer's salary is quite low regarding other tech companies' salary ranges around the Netherlands. However, the salary is considered enough to have a normal life around the country but just can't spend too much.
Therefore, if you are looking for a company to retire from because of salary, this is not a company.
If you are looking for a company that makes a real difference in people's lives and maybe have co-workers also as friends, this is the perfect company.