Answer:
Modern day anti-viruses have a few shortcomings:
1.) False detections. Anti-viruses use a set of algorithms to determine whether or not a application or library contains harmful code such as trojans or rats. But sometimes an anti-virus will flag a file it thinks is dangerous when the file is harmless.
2.) Disk space. Any file or executable takes up disk space in order to exist on your computer; anti-viruses are no exception to this. Anti-viruses may take up a large amount of space on your drive.
3.) Slows down computer. Anti-viruses are constantly loaded into memory to protect your computer. Due to this, they impact your computer's performance.
4.) Don't always work. Any dedicated virus creator will try find a way to bypass anti-viruses and sometimes may succeed. This makes anti-viruses completely useless when it comes to viruses that can bypass them.