you need to keep in mind:
never invest/lend what you cannot walk away from and afford to loose. come up with that number and don't deviate.
This statement is a little too generalized. It really depends on the type of investment.
An investment into an asset such as real estate is generally considered safer. I would argue that it’s safe to borrow money for such an investment, but only if it’s your personal investment, not giving it to someone who claims to be buying a property.
Any investment not backed by hard assets is risky. You should never invest more money than you can afford to lose into stocks or start-up businesses.
Buying groups land somewhere in the middle. Based on posts in this thread, I will definitely agree with you that it’s best to err on the side of caution and only buy what you can afford to lose, or at least what you can afford to pay off within 1 year if something catastrophic were to happen.