Insubordinate
Second mortgages/secured loans are very similar to the original mortgage except for one key difference - they are insubordinate to the original mortgage. This means that the original mortgage has a greater priority over your home than the secured loan does.
What this means is that if you default on both loans and your home gets repossessed, the lender that provided you with the first mortgage gets priority over your repossession. This means that the second mortgage lender actually has more risk even though they are also securing the loan against your home because the second mortgage lender does not necessarily get to repossess your home if you default.
The will cause a higher interest rate, some stricter loan options, and several other things that can make a secured loan less competitive than a bigger mortgage.
Some Benefits
One of the few benefits, however, is the flexibility with things like loan terms. Loan terms with mortgages are fairly strict, spreading out about 30 years, while secured loans can be faster (5 years) or longer (20 years) and can be changed to reflect how much you can currently afford.
Beyond that, secured loans and second mortgages do not differ greatly from larger mortgages. It may be fitting for some people to want to take out a secured loan later in order to have that flexibility. But if one can get a bigger loan instead, one might want to take out a larger mortgage because the interest rate should be lower and you will only have to pay back a single lender with your monthly payments, which is always an advantage and much easier to handle financially.