It is reasonably safe. As stated it is only as good as the person writing the script. I know personally I use this for a lot of tasks at work. The most notable thing I can think of it is because of AHK saves me like 3 or 4 hours each week doing a report that automates a payment application. The information I gather I first use Excel to verify it is correct, then I run it loose and I rarely supervise it. But again you must trust whoever is putting the scripts on the computer.
I'd like to think that AHK has a good enough reputation that those who have spent countless hours working on the site and code would be beyond hiding the source. The community in general is amazing-- I can't think of a specific user who has really rubbed me as the type to misuse ahk to an extreme. There are some pranksters in the community but nothing serious damaging enough that I'd say avoid the forums.
Just take a look around peoples posts and you'll see that the people here are friendly and willing to help.