I heard that geothermal with horizontal installation will fail in the summer if there's a drought.  Is this true?  If so, what could prevent this?  I live north of Dayton, Ohio.

I am in RI. When my horizontal loop was installed the installer ran PVC pipes with holes every 2 ft or so.

Each of the 5 trenches got a drip line installed about 18"- 24" above the straight out & back loops.

I have a garden hose attachment coming up out of the ground near the foundation.

We have not had drought conditions here but I still have run water into the drip lines the last 2 summers.

Seems like good insurance. Would be interesting for someone to measure COP with and without the added soil moisture.

It depends on your soil conditions. usually horizontal systems are buried 5-8 ft deep, and the soil does hold a lot of moisture there, which at that depth does not evaporate. Once you go to that depth, in Dayton Ohio, you should not have issues whatsoever. Ohio is still heating dominated, and the loops are usually sized for heating. Cooling loads are usually much less than heating loads.
Also, the term failure might not be appropriate. Could it be that your loop temps raise a few degrees higher than what you see in a wet season? sure, costing you a couple percent of efficiency during peak season. But failure is due to drought is unlikely, especially in Dayton Ohio.