I think the main difference between countries is how much the system is built purely on a pay-as-you-go-system and how much is built on a funded system (Nordic countries are succeeding in this transformation quite well, although there are still challenges ahead), many Central European and Southern European countries rely on pay-as-you-go systems mainly and the system is not sustainable (see e.g. Mercers report https://www.mercer.com/insights/investments/market-outlook-and-trends/mercer-cfa-global-pension-index/ ). In the countries where reforms are not done, the system will not necessarily implode but pensions will be more and more financed additionally through tax money (which might still be economically doable, but it can be doubted whether this is politically and socially acceptable in the long-term)