Downsides to staple up radiant heat, besides cost?
Here are some potential downsides, besides cost (which is a big one):


Can't have central air conditioning.
How to heat the basement? I assume you plan on insulating under the PEX tubing to make most of the heat go through the floor. This makes it tough to heat the basement, if that's something you want.
Will the warm floors be noticable? For well-insulated and relatively airtight homes, the floors generally won't get warm enough to notice any improved comfort, on most days.
Slow response to weather changes - Plate-and-tube less so than in-slab tubes, but this system type can be slow to respond to fast weather changes (see p33-39 in http://www.duluthenergydesign.com/Content/Documents/GeneralInfo/PresentationMaterials/2013/Day1/hydronics-siegenthaler.pdf)
"Furnaces dry out the air" is a myth.  You didn't specifically mention this, but it is a commonly held belief that forced air heat dries out the air more than radiant heat. This is a myth. See: bookstore.ashrae.biz/journal/download.php?file=ASHRAE-D-AJ11MarHVAC-20110305.pdf