What is the result of restraints on alienation on Life Estates and Future Interests?

When assessing attempts to restrict the ability to transfer an interest in property, the law is generally okay with restrictions on life estates because they are inherently not that valuable (few folks want to buy an interest in land that dies with the person to whom it was conveyed). Similarly, reasonable partial or promissory restraints on fee simples are okay.

But don't you dare try to enact a disabling restraint.