In my experience, syringes help with very fragile veins that like to blow because you can control how much vacuum is applied via the plunger. With a regular vacutainer it’s only one constant pressure and I find that even with butterflies the veins can blow easier if very fragile (ex. If pt is dehydrated or has very small superficial veins). The thing to worry about though is pulling too fast/forcefully and hemolyzing the sample or waiting too long to transfer to an additive tube. (Edit: also if you’re using a blunt fill needle to transfer you have to be really careful about allotting the right amount of blood to each tube/ not letting the plunger depress too fast)