I use kratom to curb cravings for H and it works wonders. I never take kratom and think "man I wish I was on H instead". Sure, it's nowhere near the same high (kratom feels a hell of a lot more mild, but I've only ever done smoke shop kratom, I've heard the top shelf stuff you can find online is likely to be a hell of a lot better), but I'm pretty sure it hits the same receptors in the brain and that makes it stop the cravings for me personally.
As for cross-tolerance, that varies from person to person. I haven't used H since I started using kratom so I have no clue whether there's any cross there. Some people report a cross-tolerance developing and others do not. Probably has a lot to do with frequency of dosing and amount you take per dose.
You do want to be careful regularly dosing kratom, though. Some people do become dependent on it and suffer from withdrawal symptoms similar to other opiates/opioids, like heroin. IF you're gonna use kratom, keep your doses to like 4g to 6g a day. Less is more with kratom and you just need to make sure to take enough to feel it. If I take anymore than 6g at a time I start to get nauseous and develop a headache that lasts until I start coming down.
Maybe so, but still, when it comes to things that can possibly do you harm, it's better not to gamble, yeah?