If you make voting compulsory for registered voters, you end up further suppressing voter registration (specifically suppressing registration of those in situations which leave them disproportionately unlikely to be able to get to vote and poor enough to see the fines as an issue)
If you ensure all voters are registered, then making the voting compulsory is redundant.
There is no advantage to compulsory voting, unless you believe that democracy is enhanced by diluting the votes of those with a preference for a particular representative with those that don't care.
Compulsory voting's advantage is giving people an excuse to vote.
If you are legally obligated to show up at the voting booth, even if you elect to spoil your ballot at that point, your employer can't fire you for taking off work to do so.
A public holiday would be a far less authoritarian way of achieving the same end. Postal ballots and the option of extending voting hours also exist.
And frankly, if people are at risk of being fired for taking time out to vote, the problem is that employment law allows people to be fired for voting, not that electoral law doesn't compel them to choose between the risk of being fired or the certainty of being fined.
If you ensure all voters are registered, then making the voting compulsory is redundant.
There is no advantage to compulsory voting, unless you believe that democracy is enhanced by diluting the votes of those with a preference for a particular representative with those that don't care.