What are the advantages in investing in luxury watches?

I feel, though, that ETFs are less difficult to handle...

If you are able to keep the watches in a pristine manner and watch out for supply, demand, trend and craftsmanship then yes, the price of the luxury watches you have will help diversify your investment portfolio.

Supply: Limited pieces are highly sought after; certain collaboration between famous and reclusive artists even more so.

Demand: Linked with supply, but generally knowing the likes of the collectors helps identifying what models are worth keeping long term for value appreciation.

Trend: Watches comes with different design face, functionality, materials and for some even the way it is put together is an art. Know what are the kinds that appreciate the value by being the more outstanding and everlasting ones.

Craftsmanship: craftsmanship is an dying art and one that people do appreciate paying more for it. Look for one that is unique.

To take note though, certain watches are susceptible to dust and on top of that SG has high humidity. Hence if investing in watches do watch out. Also, being physical commodity take note to keep it somewhere safe.