It mostly comes down to when you want to be able to use the invested money and whether you're hitting the (relatively low) contribution limits.
The tax-advantaged accounts can only be used for retirement unless you want to pay a fortune to the IRS in penalties (which would, of course, defeat the purpose of using a tax-advantaged account, since you'd end up ...
You have certain limit in Roth IRA contribution and it will not be sufficient to
manage your retirement, with the Roth IRA growth alone. You have to
save and invest as much as possible. There are people who save 70% of
their income and invest.
For every investment liquidity is a major factor. The invested money
should be available to you in case you need it....
There's a limit to how much you can put in a tax-advantaged account. Even with mega back-door and other ways to move money around, there's still limits into how much you can contribute
There are income limits that reduce or even prevent your ability to use some tax-advantaged accounts
Retirement accounts are designed for retirement. If you ...
TL;DNR version: Yes, money received as a gift can be contributed to an IRA (whether Traditional or Roth) but to be eligible to contribute to an IRA for any year, the IRA owner must have received compensation (essentially, money earned by the sweat of one's brow) during that year.
The source of the money (e.g. wages, savings, gifts, cashing in US Savings ...