Ranked in these QuestionsQuestion Ranking
Pro Beautiful interface
Some greeters such as the Unity Greeter look absolutely beautiful on LightDM.
Pro Theme variety
There are a wide variety of themes available for LightDM, from console-like UIs to ones that utilize webkit2 to create flashy and dynamic login interfaces.
Pro Best balance
Best interoperability, best balance between functionality and bloat, simplicity and candy, not distribution or GUI-tied.
Con Not really lightweight
How lightweight it is depends on the used greeter, but they all require much more dependencies than other lightweight greeters like XDM or SLiM.
Con Poor/missing documentation
Con Feature creep
Most people don't need (or even know) all features of LightDM.
Con Autologin never worked
After 4 years of using LightDM, I never managed to make it autologin my user in the system and there are always problems with the graphics driver. Crashes too often and requires manual restarting which is dull. Not to mention the developers update it once in a leap-year.