You do not mention your application. If your application however happens to be potable water, I know AWWA Manual M11 provides at least some basic design procedure in that realm with formulae for making 90 degree as well as other angled outlet connections like 45 degree or other angle wyes by fabrication to large steel water pipelines. Many services per this reference (particularly when employing lighter pipes and higher pressures) require reinforcement that is available in many forms (perhaps this could be used in any case to compare your results?)
I believe you are correct in your intuition that at least the most simplistic wyes (for example running a certain thickness of outlet pipe into a certain thickness of parent pipe at say a 45 angle) are different in at least a couple localized stress respects to similar fabrication 90 degree outlets. One respect as has been stated is the amount and shape of metal removed from the parent pipe/intersection, and I believe another arguable point is a different stress concentration geometry at least in the local area of the more acute groin or crotch area intersection angle of the wye. While perhaps there have rarely been problems with bursts of such specials or fittings in general, I think I have read many years ago that early in the last century (when there was little available information relating to design, nor test results, and certainly not FEA/sophisticated software etc.), some of the very early problems with at least some early (more brittle?) material fittings were encountered preferentially at wyes. This does not necessarily mean that there is anything wrong with modern codes designed around modern materials and welding processes etc., or for that matter anything said by those who write them, but it would appear to be at least an indication that in general large wyes (and/or what comes to bear on them in operating pipelines?) could be in general relatively more vulnerable from a stress standpoint than many other fitting configurations and thus should probably be carefully constructed.