Pensions can be a good tool, but the government can force participation or force participants to keep money in them when they may have a better use for the money.
Here are some downsides.1) You may not make it to a pensionable age. In which case its just money for someone else.
2) The money is locked away, so you can't buy property or start a business.
3) Money may not be worth then what it is now when you come to take it out. ( ratio of workers to dependants in the economy will send wages sky-rocketing, also resources are in higher demand from population growth)
4) Money is not mobile if you wish to live in another country.