No doubt Chapman is a basket case. That's why he was on the block to begin with. A guy with this kind of arm doesn't become available via trade at such a deep discount without off the field issues. We'll see how he does in New York. He's certainly not the first guy with off-field problems to play in New York, nor will he be the last. The constant media scrutiny will either destroy him or playing in pinstripes will take his game to the next level. There's no in-between here. It's certainly a risk, but at such a minimal investment, the Yankees would have been foolish to pass this up.
Given a choice between the two, I'd rather have Kimbrel. No question about that. But at the cost and knowing this is essentially a 1-year trial run before Chapman hits free agency (which will recoup a draft pick in return for the Yankees), it's not that bad of a risk.
Either way, the Yanks still need to address their SP. If that means moving Gardner or liquidating some of their minor league chips, then so be it. You can't have a 7-8-9 bullpen like this and not go all in.