When comparing log4net vs Serilog, the Slant community recommends log4net for most people. In the question“What are the best .NET logging frameworks?” log4net is ranked 2nd while Serilog is ranked 3rd.
Ranked in these QuestionsQuestion Ranking
Pro Supports Multiple Frameworks
Has all the features you'll most likely need, and is quite configurable. Basically what you'd expect from a very popular and widely-used framework.
Pro Modular and Extendable
Has plugins for most of the things you'll need to get done without having to deal with too much unnecessary functionality.
Pro Widely Used
Using log4net is a safe bet - many other open-source projects use it as well and it'll be just another dependency you don't have to worry about. This also means finding support to fix bugs is a lot easier!
Pro Can log structured data
Pro Multiple extensions
You can easily configure it to log into Console / File / Seq / ElasticSearch / etc.
Pro Is portable between recent .NET platforms
Pro Flexible configuration options
Configuring directly or using appsettings is fairly and there are many examples available.
Con Dealing with errors can be a pain
Because of the poor documentation and difficulty of configuration you might end up spending a lot of time just trying to fix things that aren't working.
Con Confusing Documentation
Documentation is quite difficult to navigate through unless you know exactly what you're looking for.
Con Documentation is distributed among plugins
Determining what logger, formatter, option is more difficult because documentation is associated with each component.