Yes, a big one, as the article explains: if you have a massive number of old people living on pensions / versions of social security, but not a lot of younger people working to produce goods and services, those old people are not going to be able to survive at the standards of living they'd like and b) over-taxed young people are going to be incentived to not work or leave.

These are not good things, especially because they have recursive aspects, as discussed.

its literally a catch 22 situation, its a matter of what perspective (regarding the time factor) one is looking at.

Over population will cause pressure on the sustainability of resources on the planet, not to mention inefficient use of resources and resulting pollution.

Under population in a society will put other pressures on the sustainability of a society - more of the economic type and on a longer time frame. The pressures would be more about having a critical enough market size that can support enough jobs and hence a decent enough quality of life.