Hi
I think that a harness on a Bengal is like trying to walk a border collie on a lead. You would need to be an athlete to give a BC the exercise he really needs.
A harness is however a good optionr people in flats and for those not able to make an enclosure, to give their cat some fresh air.
If you use one with a long lead then it can get tangled in plants, bushes, trees, as cats like exploring unlike dogs, although I am sure someone's dog goes up trees!!.
The cat gets used to it especially if you let him go where he wants at the start, they soon get the idea but there is a lot of rolling on the ground at first. Best not to drag him as I think this is counter productive. The harness is safe as long as it fits correctly and is in good condition.
For piece of mind in your situation I feel an enclosure is best.
Some ideas
http://www.catsofaustralia.com/cat-enclosures.htm
They have three pages and loads of links. They are in Australia, so some of the companies are unavailable here, but they give you some ideas to give to your local joiner or if you are good at DIY yourself.
HTH
Elsa