It's only high for regular income. Belgium does not have a capital gains tax. If you start a company, it does well, and you sell it, you pay zero taxes.
Similarly, when you receive stock options after joining a company, you get taxed immediately, but you don't pay taxes when you exercise them. That's really backwards when you join an established company (because you'll pay taxes on something that might actually be worth nothing), but it's fantastic when you receive stock options in an early stage startup, with options that are worth pennies.
This has a very high risk of your associate your business with your private life in case of bankruptcy ( to pay less taxes).
Additionally, i know an owner that put his wife as boss instead of him for tax reasons. So tax optimalisation is very very complex here, another disadvantage.
Considering taxes in Belgium are the highest in Europe and almost the highest in the world.