Need confirmation: Subselects are broken with regards to index usage?

[Robert DiFalco]

I think I'm discovering that sub-selects in MySQL are broken. Is that
true? It seems like you cannot have a sub-select without doing a table
scan -- even for a constant IN expression -- this because it gets
re-written as an EXISTS that executes for each row.
 
Is that true? Forcing an index doesn't even seem to help. 
 
R.