The main drawback to entering into an installment agreement is the fact that interest and penalties still accrue on your unpaid tax bill.
The consequence of having a large unpaid tax bill combined with making small monthly installment payments is a larger tax bill.
On average, the IRS interest and penalties have a combined rate of anywhere from 9-12% per year. This can easily add up.
It is very frustrating to make monthly installment payments and see your IRS tax bill increasing. This is another reason to increase your monthly installment payment or to pay your tax bill in full.