Is it safer to run suspicious (trojan, malware, virus) software/exe/applications inside VirtualBox (Windows) and what threats do they have?
Your question differs from the one stated in your post. Is it safer? Yes, it is safer to execute programs in a virtual machine but it is not complete safe (then again, what is?). You can escape a virtual machine a vulnerability is used, in this case within VirtualBox.
You also have to keep in mind that setting up the virtual machine poorly will allow malicious code to escape the virtual machine. This all depends on you.
It depends on the malware itself what they can cause. It can vary from using your computer or virtual machine in a botnet to destroying all files on the computer.
Edit It was brought to my attention this question is a duplicate, see this answer for a more detailed answer to your question.