Participation is totally voluntary. Private ownership still exists. When someone is dissatisfied, they can seek redress within the system, and if they find that unsatisfactory they can leave with little impedance. It's a voluntary co-op, not a compulsory governmental program. While human nature will exist, widespread tyranny and/or compulsion is not a function of The United Order, and those inclined to prefer that nature over discipline or concern for the group are free to do so, and they are free to pursue whatever opportunities they would like on the outside.
Control is very local, administered by bishops who supervise a few hundred people. Unless there was blatant misappropriation or significant credible protest, a bishop's dispensations would likely go unchallenged. Broad centralized programs that cause trouble for some unforgotten little guy are not plausible in a scenario where control is so widely dispensed and so locally focused.
Of course, there's a religious element to this. We believe the bishop has the right to be led by revelation in these duties, so that dilutes a lot of otherwise intractable social ramifications of such an organization. If a man has been ordained by the prophet to do a thing, tolerance is much, much higher than otherwise, and peace is much more likely to be kept despite individual disagreements.
By extension, Mormons further believe that those who accept the judgment of the bishop with the recognition that their bishop is the authorized representative of God in that matter, even if it opposes their personal judgment or opinion, will be lead and blessed such that they become the wealthiest people on earth many, many times over, incomprehensibly more wealthy than contemporary Western nations. Mormon eschatology, in fact, indicates that a society operating under these conditions will be the only place of prosperity on the whole earth for a time, and everyone else will be at war and unable to cooperate sufficiently to sustain any significant productive output.