While A520 doesn't support PCIe gen 4 to the GPU, it does allow gen 3 to all PCIe devices even those on the chipset bus which will be all the PCIe slots 'below' the GPU and secondary M.2 sockets. B450 allows only gen 2 to devices on the chipset bus, so things like secondary M.2 NVME's will only get gen 2 at 1/2 the data throughput. That's only an upside if you plan on using it, and the board has a secondary M.2/NVME and not many low-end B450's have one for that reason.
For a 3600 processor it doesn't matter but if ever the time comes to consider upgrading CPU's to Ryzen 5000 an A520 is a natural fit with fully tested BIOS's. B450 boards will only get BETA BIOS's and may see limited future updates, so if that's an issue for you it's...