Without liquidation preferences BrewDog wouldn't have been able to get the investment. They may have been able to get a loan instead, but the interest would've driven them under that much faster and then the creditor, just like the preferred investor, would have priority over other shareholders.

Which isn't to say preference stacks (like debt stacks) can't get absurd when a failing company is doing anything it can to stay alive, but standard investor terms (1x liquidation preference) simply mean you're first in line to get your money back if the company is liquidated for less than the price you paid.

The self-dealing bit is generally already illegal and orthogonal to liquidation preferences.

Liquidation preferences that merely guarantee you get back the cash you put in before anyone else gets anything are a perfectly reasonable protection for investors.

Otherwise, the risk is investors put in $1m for 10% of the company, only for the founder-CEO to decide a month later - perhaps totally genuinely and honestly after initial R&D - that the entire business plan wasn't viable after all and the best thing for stockholders is to liquidate the company... and then walk away, personally, with $900k while returning only $100k to the investors. And obviously because this scenario is possible, there would be great temptation among dishonest founders to "realise" their companies "aren't viable" in order to walk away with all the cash. Something has to protect investors from this outcome, and remove the perverse incentive on founders to promptly liquidate after getting investment. A liquidation preference where the investor gets repaid before any distributions go to other stockholders is one way of achieving that protection; what alternative would you prefer?

As for liquidation preferences with a multiplier (i.e. we get paid back X times our investment before anyone else gets anything), eh, I wouldn't outright make them illegal, but it's less clear to me they are always serving a legitimate purpose, and they may grossly mislead the public about the actual valuation of the company implied by a given funding round AND mislead naive common stockholders and options holders about how much of the company they really own and what they can really expect to make in an exit.

Well, thats what "free markets" (tm) allow you to do (-:

I've been screwed out of company stock at liquidation multiple times. Who decide to pay never to favor the workers.

By the time most of us are dead, I think we’ll have enough evidence that human systems mostly don’t work (this is a personal realization, just for ourselves, no one will be able to strip you have the realization). If it worked for you, congrats, you were lucky.

You've got that backwards. Liquidation preferences typically go to early stage investors who receive special share classes. Those arrangements are fully disclosed to later stage investors. They knew what they were getting into and it's their own fault for making a sucker bet.