And if you are unlucky (Belgian IT salaries aren't that great compared to the rest of Western Europe) you gets bumped a tax class, possibly making those donations a net loss.
If you're at the edge of the 45% rate, the extra 100€ will be taxed at 50%, but the amount you earned previously will not be impacted and still be taxed as before.
Our taxes in Belgium are high, the salaries for engineers are well above average. In pratice, it's possible to end up in different tax 'scales', but they'll not end up in a net loss.
I'm also hugely in favour of the system here. Health care is a non-issue. I need prescription glasses (dioptry -8.5) and I pay nothing for those.
Yearly dentist appointment? about 20 euro you end up paying after the 'mutualiteit' pays you back. Similar for doctors appointments or hospital appointments.
Being married with a Mexican (family living in the US) I can tell you that I hugely prefer our system over the US or Mexican system.