I no longer have 7zip installed to test it myself, but I do know I have used it in the past to open zip and rar files, but I took this from their website:
The main features of 7-Zip (version 9.20)
High compression ratio in 7z format with LZMA and LZMA2 compression
Packing / unpacking: 7z, XZ, BZIP2, GZIP, TAR, ZIP and WIM
Unpacking only: ARJ, CAB, CHM, CPIO, CramFS, DEB, DMG, FAT, HFS, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, RAR, RPM, SquashFS, UDF, VHD, WIM, XAR and Z.
For ZIP and GZIP formats, 7-Zip provides a compression ratio that is 2-10 % better than the ratio provided by PKZip and WinZip
Strong AES-256 encryption in 7z and ZIP formats
Self-extracting capability for 7z format
Integration with Windows Shell
Powerful File Manager
Powerful command line version
Plugin for FAR Manager
Localizations for 79 languages
In their FAQ, it looks like there are instances when it can't open some archives that contain incorrect headers, where other programs might disregard errors and continue. Maybe that's the issue?