OK, I can see it working efficiently with a variable speed air handler that is controlled well enough to run continuously (so that the excessive size of the HE doesn't retain and re-release condensation). An even larger efficiency gain would be large hot/cold tanks that store enough so that you don't use the ground loop during a day of balanced heating and cooling conditions. Or zoned ground loops. Or the low hanging fruit of using different temps for radiant and air handlers.

But are such complex systems practical for home owners?

Not for the typical HO. A packaged WtoA with a dehumidifier would suffice in a humid climate that requires dehumidification. The OP and some HO's demand the comfort of radiant heat and yet are still looking for AC with WtoW. My point was at that point with two tanks you already have all the equipment in place to dehumidify.

Hydronic air handlers intended to operate with more extreme water temperatures are up sized for use in geo applications with more moderate water temps which means lots of coil surface area. We do not have comfort issues in MI with just WTW geos and hydronic air handlers.
Get rid of the engineer who thinks bigger is better. In geo that's often untrue.

Posted By joe.ami on 25 Aug 2014 12:09 PM

Get rid of the engineer who thinks bigger is better. In geo that's often untrue.

Please explain, I'm not sure I understand why.

In geo "bigger" most often means higher first cost and higher operating cost. It also suggests someone who is not geo savvy so you are getting advice from someone ignorant to design"best" practices.
Doesn't mean your friend can't make it work, simply means it will not be the most economical way and it won't be betterfor the added expense.