Well, there seems to be many opinions, many of which could be correct, however... With the proper laser they can be quite good for soldering.
Here are a few guidelines I would like to suggest. 1) Have your product soldered first (by the vendor) 2) Try to spec in a 960 to 980 nm rated laser. These diodes are more penetrative and less refelective. 3) Use a wire or paste that has a 3 to 3.5% flux content. They seem to solder better. 4) If you can get variable beam, it is a great feature. This allows you to have a range that the beam can be adjusted for various pad sizes. and finally 5) Vision is a good feature. While it costs more, it allows to you precisely target the pad each time.
Hope this helps