I've personally used a family guarantor for 2 properties. It allowed me to preserve my own funds for future purchases. First 2 properties I didn't put a cent in (besides small renovation) and still haven't reduced the total debt, just increased the debt secured against the purchase property and decreased the guarantor loan. Guarantor will be released in around 5 years from time of purchase.
This worked well for me, but doesn't mean it's for everyone. Most of the time I had the funds available to payoff the guarantor loans. 1 loan was secured by family cash (a loan from them would of worked in a similar way, but for my bank it's easy to establish) and other was using an IP of my parents.
I have fixed the loans during the period as well, but both have been 2 years and at the end of the fixed period when goes back to variable I top up at mentioned above.
Also worth mentioning that these properties weren't super expensive both <$300k properties, so gurantors were only liable for ~$110k at the peak for both properties.
If you do go down the path of family guarantor make sure you have a good discussion with the guarantors around how long until they expect to be realeased, as many times families sign on thinking they are helping, then want to make changes of there own only to end up in a little family fued.
In some cases it works well having the 80% as IO and the 20%+fees as P&I and make all extra repayments off the 20% to release guarantors.