A sobering example of the power of JT to destroy a will was when I was executor for a will that formed a trust for children and a life interest for the spouse, all clearly stated, witnessed and signed - BUT - the solicitor who drew up the will had neglected to change the Joint Tenancy to Tenants in Common, the will failed, the co-joint tennant inherited absolutely, nothing that could be done.
There was a case on here recently where a child was gifted a share of property in a will, but the property was JT, again the will failed.
You can promise anything in a will, I could draw up a will in which I gift Buckingham Palace to my cousin, that would be entirely legitimate, but unfortunately as I don't own it my executor could not pass on the asset described and my cousin would end up with nothing.
That's the issue with JT, as soon as one party dies, their notional 100% chunk dies with them, the other party already owns 100%, so there is no asset to pass in the deceaseds will, it just doesn't exist.