Vias under thermal pads
When you put unfilled vias in a pad, and then put solder paste on the pad with a stencil, the vias will wick solder away from the pad during reflow, potentially "starving" the joint with the component lead/pad. You'll need to account for the extra solder required, perhaps by making the stencil opening oversize, or by specifying a thicker stencil that will leave a thicker layer of paste behind.