If you're hosting, you can move the trees to the entrance and add a fence around it (which you're likely to do anyway) and you've minimized the risk. The most a malicious person could do - even with best friends - is chop down or steal the trees, which is a risk even if you're trading individual fruit pieces. Also, if they're acting shady, you can close the game or kick them.

If you're going to their island, yeah, they could just kick you after you plant the trees you brought, but again, trading the individual fruit doesn't stop that.

Yes, if you don't bother fencing off the airport and give a rando free reign over your island as a Best Friend, that could end very badly. But I took it for granted that people would take reasonable precautions.

Unshaken trees have a handful of advantages over fruit. They can be harvested immediately, as opposed to waiting for it to bear fruit. The person can't try to trick you with stacks (dropping 20 fruits looks identical to 2 fruits until you go over to it and look, which might give them time to grab your fruit and bolt, whereas there's no way to make one tree look like multiple trees).

There are disadvantages, too, obviously - mostly involving being less convenient - but it's not really any riskier as long as you take reasonable precautions.