For those looking to dive in, the MAME Documentation is the best place to understand why your ROMs might not be working with the latest version of the software.
As of recent versions (like v0.260+), a complete MAME set is massive. ROMs Only: Expect to need around 70GB to 100GB CHDs (Compressed Hard Disk Images): Mame Full Set Roms
Mame Full Set Roms refer to a complete collection of ROMs (Read-Only Memory) required to run every game available on the Mame emulator. ROMs are essentially digital copies of the game's original code, which are used to emulate the game's behavior on a different device. A full set of Mame Roms includes every single game that is compatible with the Mame emulator, which currently stands at over 30,000 titles. For those looking to dive in, the MAME