There are some limitations(thickness, D/t ratio) to use the template of Wye in FE/Pipe. This template may not be applicable to the case with thin walled(ex: 36” pipe, STD WT.) Wye or an integrally forged product. ASME B31.3 asks to meet the requirements the para.304.7.2 for unlisted components.
To get the valid SIF by using FEA, as John mentioned, only qualified FEA user should get accurate values. For some cases, thin walled Wye or Laterla with sharp angles, only by entering the SIF into Caesar II may not be enough to verify these components.

In my opinion; if your pipe is major system with high pressure, you’d better let vendor design it and you provide your design conditions including loads. Some vendors such as WFI and GCC provide the SIF with FEA and busting test. This case will result in not only the higher engineering and material cost but also delay of engineering schedule.
If you piping is not a critical system, use very conservative values…..