Igor,
Other than liability issue, my biggest concern is damage to the trees.
I have a few acres of lemons and 25 years ago, used to invite friends and neighbors to help themselves when in need of some lemons. But, then I realized the overall damage they were doing. In addition to breaking new growth and irrigation, etc., damage to new blossoms became intolerable. You must cut or snip the fruit at harvest! Yanking or pulling fruit off the tree, ends up shaking off many new buds or blossoms. So ended the open invitation to U-Pick! My final solution was to pick and keep on hand a few boxes of lemons for giveaways. Just sayin…..
Also……. good point about potential for new pest/disease introduction. Never thought of that but, sure my grove manager is all over that issue.