You need to keep a close eye on the promotional period.
As soon as it expires, normal interest rates will apply.
The standard variable APR for Citi Platinum purchases (8.99 percent) will be applied to all remaining purchase and balance transfer amounts.
Likewise, the standard variable APR for cash advances (19.99 percent) will be applied to all remaining cash advance amounts.
If you default on Citi Platinum's card agreement, the company can immediately increase the APR on all balances including any promotional balances to a variable default rate of 28.99 percent.
Your post-introductory APR will depend on your credit history.
If this interest rate is significantly higher than the rate on your old card and you have a remaining balance, you'll wind up losing money.
Of course, you could always transfer your balance to a new card with a lower promotional rate.
Just be careful not to entangle yourself in a vicious cycle that could backfire later.
Good luck with your credit card balance tranfers.