I think bonds are fine as long as they are transparent and both parties go in with eyes open. If you intend to stay with the company long term, then you should not be negatively affected. It sucks if an opportunity pops up during your bond period, but that is where the eyes open part comes in. Know what you sign up for.
However, the companies that ask for both a bond and a salary deduction for incurred costs, those are the ones to avoid. My previous employer played around with such schemes. I joined as experienced pilot, was bonded for 3 years but with full salary. Some other who joined after with less or no commercial experience, were bonded for the same 3 years but with a lower salary as well.
When I went for command, I started the command course with a deal for another small bond, which ran along side the type rating bond. It basically extended my time commitment by 6 months. All good. When I was done with the command course and got the contract to sign, they added a salary deduction which would have me sit in the left seat for less pay than I had in the right seat
. I told them I would just continue flying in the right seat then, after which it quickly went back to the original deal.
But yeah, scummy companies will try. And there are those who will accept those conditions as well.