Every protocol for GeCKO library says to use electroporation transformation method but not chemical transformation. Moreover addgene protocol says not to use chemical transformation for library amplification. Why you recommend not to use it? Because of low transformation rate or there is another cause?
We usually recommend electroporation instead of chemical transformation for amplifying the sgRNA library because electroporation is much more efficient, i.e. more colonies/ng of DNA. For an sgRNA library where you need a large number of colonies, and just enough DNA to start (when you get the library from Addgene, for example), electroporation is recommended.