In general, I don't think there is anything wrong with a lockback design, IF that's what an individual likes/desires. The reason that I do not produce them is simple......safety. To explain: When/if a lockback locking design fails, it fails suddenly, immediately, and usually with catastrophic results (someone usually gets hurt/cut badly).
In my opinion, a liner lock is a much safer design, in that... in order for the locking design to fail, the locking bar must either crinkle or break to the point where it can no longer make contact with the locking area of the blade. (I've never seen or heard of this happening on a liner lock) Since the lock bar is "sandwiched" between the handle scales, it's a virtual impossibility for it to fail suddenly/immediately. When a liner lock fails, it does so slowly, meaning that it gives the user plenty of warning that something isn't right. Because of this characteristic, it means that IF a liner lock fails, it almost never causes injury to the user. That's not to say that it's impossible for someone to be injured due to a liner lock failure, it's just the the odds of NOT being injured, due to a liner lock failure, are MUCH greater.
As Steve mentioned, Liner Locks are also easier for a maker to correctly produce, simply because there are fewer parts, and fewer precise angles on the parts. A liner lock (depending on the individual maker's style) will have only 1 or possibly 2 surfaces of the locking area(s) that require specific, precise angles. With a lockback design, there are no less than 4 surfaces that require specific/precise angles. Finally, most folks can easily learn to operate a liner lock design one handed, which makes the design more "usable" to most. User care/Maintenance is far easier on a liner lock.....generally just about anyone can take a liner lock apart (screw construction), clean it/lube it, and then reassemble it in such a way that the folder works. That is usually not the case with lockbacks.
There are still some folks out there who believe that lockbacks are a better design, and that's OK. "Better" is such a subjective term. In the end it's up to each individual to weigh the pros/cons of each design, and make decisions based on those insights.....what I have talked about here are only MY opinions, based on MY observations and experiences.