Investments that promise high returns usually make a lot of very hopeful assumptions about the future. When you have interest to pay as well, you are putting a lot of faith in those assumptions. You should check the documents very carefully. You do end up paying for the guarantee through a higher interest rate or some other device. You should also remember that even if you do get your original money back, you have still paid interest in the meantime on what you borrowed.
Loans usually cost money to set up.
Application fees, valuation fees, account keeping fees. Sometimes the salesperson will also get commission for setting up the loan with you. You should find out about all these fees and about any commissions. You also need to check about repaying the loan. Sales pressure Tax schemes are usually sold as the end of the financial year approaches.