What are the main advantages of UGMA/UTMA? Under what circumstances are they good or bad?
An UTMA account is basically a taxable brokerage account in your kid's name. You have control over investments and distributions until an age specified by your state (usually 18 or 21). After that age, all legal control over the account is transferred to the kid, whether you think that's a good idea at the time or not. Before that age, you're allowed to withdraw the money for the kid's benefit, but are not supposed to spend the money on anything that you as a parent are required to purchase from your own funds (food, housing, clothing, medical care, etc.). The theory is that once the money hits the UTMA account it is no longer yours; it is an irrevocable gift to your child. You can't make a minor child pay for their own room and board, so taking money from the UTMA to pay for that is therefore out of bounds.
As the money belongs to the kid, a large UTMA balance upon entering college can be very detrimental to the kid's chances of qualifying for need-based financial aid. Students' assets tend to be weighted much higher in financial aid formulas than parental assets (a 529 generally counts as a parental asset).
Where the UTMA can be useful is that a kid's investment income below the kiddie tax threshold ($2,100) is taxed at the kid's own rate, often 0%. Suppose you transfer some appreciated stock to the UTMA and realize $2,100 of capital gains within that account instead of your own. The kid will be paying 0% federal tax on that, while you might have paid 15% or more. Seems like a win, as long as you can keep the overall balance low enough that you don't run into some of the downsides (the prospect of handing over a bunch of legally unrestricted money to an 18-21-year-old, lower financial aid in college, etc.).
How to keep the balance down after making a gift? I think basically any discretionary expense for the kid would be allowed here. Summer camps, computer equipment, plane tickets to visit out-of-state relatives, music lessons, and sports leagues all seem like defensible expenditures: something for the kid's benefit that a parent isn't legally required to provide. I haven't yet opened an UTMA for my two-year-old, but am considering doing so this year now that I have some taxable assets that have appreciated a bit. The prospect of up to $2,100 in tax-free capital gains seems worth a little bit of hassle in setting up the account.