Two broad designs exist. Deferred accounts give relief on contributions and tax withdrawals; exempt accounts take after-tax contributions and pay out free of further tax. Which is better depends mainly on the rate now versus the rate expected later, a comparison nobody can make with certainty.
The shelter changes what belongs inside. Because gains and income are not taxed as they arise, high-turnover strategies and heavily taxed income such as bond interest or REIT distributions are natural candidates. See asset-location.
Restrictions are the trade-off: contribution caps, penalties for early access, and in some regimes mandatory withdrawals later in life. Foreign investments held inside these accounts may also lose treaty benefits on withholding tax. See withholding-tax-drag.
Related: asset-location, tax-loss-harvesting, withholding-tax-drag, reit, cost-basis-method, safe-withdrawal-rate