In short: However, there are three niche scenarios where a user might deliberately seek out the old DX9 renderer:
Currently the gold standard for performance and accuracy across both NVIDIA and AMD GPUs. It is highly recommended for its efficiency and is the default for most modern setups. pcsx2 directx 9
Maintaining the aging D3D9 code was preventing developers from implementing advanced fixes in newer renderers. For example, adding depth sampling to Direct3D 11 was incompatible with keeping the D3D9 backend. In short: However, there are three niche scenarios
A tiny handful of games—specifically obscure Japanese titles or demos—never had their DX9-specific hacks ported to DX11. For example, Steambot Chronicles had a unique shadow glitch in early DX11 that worked perfectly in DX9. These regressions are rare (less than 0.5% of library). Most have been fixed in Vulkan. For example, adding depth sampling to Direct3D 11
Do use PCSX2 DirectX 9 for action games, RPGs with heavy cutscenes, or any game that uses post-processing. You will encounter half-screen lines, texture corruption, and random crashes.