Not completely true either, it eventually supported most of the normal 3d primitives but gaming performance was never a priority because there were few developers and they weren't employed by AMD/ATI -- which also meant that some cards would only reach full feature support after their EOL, sadly.
The amdgpu also driver benefits from a lot of the groundwork that has been done since. The radeon driver is older than kernel features like KMS (kernel modesetting) and GEM (graphics execution manager), and the LLVM-based shader compiler in mesa (userspace). I'd say that the radeon driver was actually the proving ground for many of these features, because it was the most capable open source 3d driver: The Intel 845/915 hardware barely supported 3d operations, and the only 3d-capable open source driver for Nvidia was the reverse-engineered nouveau driver.
Luckily, many people working on the amdgpu driver are actually on AMD's payroll these days.
I forgot that name "fglrx", probably a mental self-defense mechanism. Those were some bad times, trying to get different display outputs to work at the same time, guessing and testing values in xorg.conf, so on. There was some community utility someone wrote to try and help with installation, reinstallation, configuration and reconfiguration, but the name eludes me now.
I would edit my post to correct it, but it seems the edit window has passed.
Even Sea Islands/Southern Islands were much better with amdgpu (but you have to use a module parameter to enable support).
Ah and hardware video decoding never ever worked again.
So much for the so called advantages of an open source driver.
Once the rewritten "amdgpu" driver came out, things got much better. The first few cards created after that (IIRC the Polaris GPUs, RX 400's), the situation reversed. I still have had occasional issues with various Nvidia cards (normally driver updates breaking things), but for almost a decade now, I have not had issues with AMD GPUs under Linux.
[0] Except for pro features while using workstation cards. You need to use a proprietary driver for those, but even those share a lot of code with the open source driver.