I was just wondering I will be using their code in my site, right?
Are they perfectly reliable? I mean hacking and other malicious activity wise.
Are they perfectly safe? I don't know as I don't have knowledge in programming languages.
But I also haven't seen a complaints or any user raising these type of issues.
Bootstrap is used by 26.6% of all the websites whose JavaScript library we know. This is 21.7% of all websites. (as per sources)
so this will give you idea how reliable it is!