There's a flip side to everything, and in the case of Open Source software it all boils down to the old saying of "there's no such thing as a free lunch". Most of the disadvantages only apply if you're not somewhat code-savvy and willing to get your hands dirty:
Projects can die:
Just as with commercial applications, some Open Source projects stall and die - programmers just lose interest with being involved or they become fraught with infighting; arresting further development. You may be left using an application that is full of security bugs, with no-one to fix them; or if you can get them fixed, it will usually require paying someone to do it.
If you are using commercial software, the vendor has an obligation to assist you in a timely manner, especially where security bugs are involved. You may find an annoying bug in an Open Source application that you need assistance with, but you may not get it without paying someone to fix it.
Even with general questions, if you take a look at many forums that act as support centers Open Software applications, you'll see that it's not unusual for questions to go unanswered. The Open Source community does not have a legal obligation to answer your questions. In some cases, you'll need to figure it out for yourself or hire the services of a knowledgeable contractor.