1) It isn't a complete solution: Most of the time the antivirus portion is there but none of the other components like firewall, browser guard, phishing protection or spam protection is there. If you search hard enough you can find a free program for each of these but you would then be running 4 programs instead of one.
2) Many free programs have to be manually updated. If you don't remember to do this your protection will be outdated in a day or two. You may only remember after it is too late.
3) Most of them don't scan on their own so that, once again,.there is something you have to remember to do.
That being said, the free software is better than none. If you need help getting something on your computer let me know.