Does it have any spyware, viruses, trojans, or other malware?
No; though some anti-virus products (Kaspersky) raise “Trojan.generic” warnings due to the auto-uninstall feature, these are false positives. Just make sure you obtain UNetbootin from the official downloads page on Sourceforge not some shady third-party source. If you’re absolutely paranoid, you can check the source code and compile it yourself. Certified by Softpedia.