I think most are capable of understanding it. I use it every chance I get.
Once, I popped into a KH parking lot about the time I knew some would be returning from Field Service on a Saturday morning, around 11:30 or so.
Using only this information, after about 5 to 8 minutes, I was able to get an elder to make the following statement:
"If what you are telling me is true, then I'm in the wrong place".
I have had many JW's (and family members) appear dumbfounded when simply asked to show even one scripture in the NT that shows how a person can get their sins forgiven while outside of the New Covenant.
The key is not to preach to the JW, but to ask them to instruct you about this supposed salvation outside of the New Covenant. Of course it doesn't exist.
This is the entire purpose of the Watchtower, to get people to turn down the FREE GIFT of pardon. If someone is bound and determined to turn it down, then so be it. But most JW's are utterly unaware of what they are choosing. If we love them, we will help them to see what their choice actually involves.