As Bronte has pointed out, I don't think that lenders are doing them anymore, so it's probably a moot point.
Guarantees have received a lot of bad press. Guarantors have claimed that the lenders told them that the guarantee would never be called in.
However, they have a valid role and should be considered. Credit Unions use them a lot where they are lending to a new person - an existing member may be asked to guarantee the loan.
Buying a house with borrowed money is much cheaper than renting an equivalent house. If the buyer needs a guarantee to get the mortgage, then it's well worth considering. But the guarantor should understand that they are taking a risk and should not guarantee it if they have any doubts about the borrower.