Here are two potential fixes I have found for some of the errors that caused my UEFN to crash regularly.
For “not enough VRAM” problems or shader compilation errors, switch both your UEFN and Fortnite launch options to include the flag “-d3d11.” This should switch it from DirectX 12 to DirectX 11, which is much more stable.
EXCEPTION_ACCESS_VIOLATION reading address 0x0000000000000000 was a niche problem for me, stemming from how MSI motherboards and new Intel CPUs interact with each other. It also affects Asus motherboards. To fix it, you need to underclock the CPU a little bit. In my case, I changed it from 5.4 GHz down to 5.2 GHz, and this completely fixed all UEFN-related crashing for me.
Hope someone finds this helpful.