One potential problem of sending long range rocket systems to Ukraine:

 What if someone really irate in Ukraine decides to use a missile or missiles from the U.S. and targets Moscow? What if this happens?

In other words sending a long range  missile system to Ukraine could have unpredictable consequences if the wrong person got a hold of it and actually started World War III by accident.

Or what if Putin decides to do a false flag attack on Moscow and blames it on Ukraine and the U.S.?

All these things are possible things that could go wrong when giving long range missile systems to Ukraine.

We want Ukraine to defend themselves but what if people who use these missiles are having PTSD from what happened to their grandmother, aunts and uncles and parents and children or wives? 

Later I was just checking how many kilometers it was from Kharkiv to Moscow. If these missiles have a 500 kilometer range they could make it not quite to Moscow if launched in Kharkiv which might be important to know.

Google maps said that it is almost 742 kilometers to Moscow from Kharkiv. However, many other Russian cities including Belgorod, Voronezh, and Saratov would be within range of the 500 mile kilometer range of U.S. missiles that U.S. might give to Ukraine.