Liner locks were first invented and used in their present form by Michael Walker, a very famous high-end custom knife maker. Done properly, it is a very safe locking mechanism. Made poorly or wrong, it is next to useless. It is probably the most common locking mechanism used in folding knives in All prices ranges, both production and custom.
There are many other variations of the locking folding knife: integral side bolster locks, the Axis lock, the REAKT lock, the traditional back-lock and front-lock, Bill Peases's front side lock, and Scott Sawby's 'self-lock', just to name a few.
There is nothing inherently wrong or bad about liner lock designs. But they can be done well, or poorly. Putting trust in a poorly made liner lock mechanism can risk fingers. But that is true for Any folding knife lock. Some are better than others.
I think that liner locks are fine for most purposes, every bit as good as a lockback, IF it is a well made knife.
As has been said here many times, all folding knives are susceptable to failure. I think that liner locks may be a more difficult design to execute, so...stay away from the cheaper models. Very good liner locks can be found in the $50.00 range though.
Personally, I prefer frame locks, they are simple and strong enough for my needs.
Up until recently I would have warned you away from liner locks. I won a Spyderco Military here on bladeforums a little while back, the liner lock as well as the whole knife impresses me greatly. I didn't know liner locks could be like this. A well made liner lock is fine for most purposes. I still love my Rolling lock though. I wouldn't worry too much on the locking system. Find a quality manufacturer and knife design you like first. Merry Christmas
I am not a big fan of liner locks. Besides being generally, and justifiably, suspicious of their strength - they are just too finicky. They are tricky to get the right fit at the factory, they are prone to wear and can get gunk in them with messy use and may not unlock easily. I like the SERE 2000 and will put up with it in that knife, but it's really not my favorite lock.