The main problems with 100% mortgages are higher product fees and interest rates.
Lenders can make you pay a higher lending charge, which is a fee for borrowing with a small deposit. This gives them additional protection in case you miss payments or you fall into negative equity.
Guarantor mortgages are risky for the guarantor: if you fall behind on your monthly repayments, your guarantor must pay the lender instead. If they cannot meet the payments, they could lose their linked savings or have their home repossessed.
The other risk with 100% mortgages is that you could end up in negative equity if the value of your home decreases. Negative equity is where your home is worth less than the outstanding mortgage loan. This means the lender wouldn't get their money back if you sold the house - which is a bad situation for both you and the lender.
If you're in negative equity it can be hard to remortgage to a new deal. You could become a mortgage prisoner with large monthly mortgage repayments.