Control, tax treatment, and impact on financial aid are key advantages.  You may intend for the monies that you have placed in a UGMA account be used for higher education, but the control of this account transfers to the beneficiary when that child reaches legal age. If he/she wishes to use those monies to follow a surfboard's summons into the sunset, you have no legal control - the funds are then controlled by the would-be student, now a legal adult.  With a 529 account the ownership does not automatically change and the owner is always in control.  

Contributions to a 529 may be deductible for state income tax purposes.  Account earnings in 529 Plans grow tax-free and may be withdrawn free of federal and state income tax treatment when used for qualified education expenses.  UGMA accounts do not receive that favorable tax treatment.  There is no possible deduction against state income tax liability for contributions to an UGMA and withdrawals aren't tax-free.  Investment income above $850 within an UGMA is taxable. 

Starting in 2006 funds in a 529 Account are not considered when applying for federal student aid.  However, assets in an UGMA are counted as an asset of the student.  Monies in an existing UGMA can be rolled into a 529 account and receive the positive treatment for tax and financial aid considerations going forward.  However, the new 529 account must retain an UGMA ownership registration and the account will become fully controlled by the beneficiary when they reach legal age.  All 529 programs do not permit an UGMA 529 ownership registration.  UGMA/UTMA accounts are governed by state statutes.

UGMAs have a few pros over 529 accounts.  The range of investments that can be held in UGMA accounts is very wide and there is no limit on the frequency with which investment changes may be made.  Withdrawals not used for college do not incur a penalty.  All assets in UGMA accounts belong to the minor for which they are being held and must be used for that minor's benefit.