There are areas fairly close to the Airport that are definitely to be avoided.
However, if you stick to the interstates and main roads you will be okay.
Miami have done a lot over the past few years to improve security and I think providing you do not stray too far away from the main road links you wont have any problems at all.
I think its only poor tourists who are unaware and drift into no-go areas at the wrong time of day that have problems.
To be honest most major cities have areas not too be around at night, parts of Orlando are to be avoided also.
The rental car collection is not ideal as its a bit away from the actual airport.
Fort Lauderdale/Palm Beach and Boca Raton are all very nice, its a very pleasant drive down the coast on the A1A route and well away from any trouble spots.
It's only the 4 hour drive to Miami that puts us off returning, we went on honeymoon there and have only been back once since. Our young son loves Orlando and wouldn't get anything out of it really. For shopping Miami is the best by a mile.