I've compared
ServiceStack ILog ,
Common.Logging/ILog and
NLog/Logger.cs
and found that NLog has bigger number of overloads, that make it more type-safe. Also NLog has logic to not process formatting if the logging for the particular record is disabled.
Based on this I decided that using generic logging interface is YAGNI. If I would write library to be used in multiple applications with potentially different logging library used,
ServiceStack is a way to go.
However in application development with no immediate need to support more than one logging framework, it is more efficient to call Nlog directly.
If in a future it would be decided to switch to other provider, global replace changes to call
ServiceStack
or directly new provider should be relatively easy.