But both ratio and time heavily depend on if there is data in the file that can processed by Precomp. Since deflate/zlib/zip/gzip is used for other filetypes and containers (.docx/.apk/.jar/...) and stuff like game resource files, so results can be surprisingly well, especially when comparing to traditional compressors that can't handle the compressed data at all. On the other hand, newer compression methods like .xz/.7z/.rar/brotli) aren't supported (yet), so Precomp can't do much about them.
Not being backward compatible is more of a protection in alpha phase and to avoid confusion, the error Precomp gives when trying to process files created with older versions could be changed to a warning. In most cases, the version check could be patched out and the data of older .pcf files would be restored correctly with newer versions.