Successful Criminal is a contradiction of terms 99% of the time.
There seems to be a sweet spot for criminals.
For example, you have to be powerful enough that nobody will turn you in or turn on you, plus you have to be low-profile enough that the bigger crime lords won't mess with you nor will law enforcement. To an extent, anonymity can play a role - I seriously doubt the corner boy hustling drugs on a street corner knows who the supplier is, so if they're arrested, they can't really turn in the big boss for a reduced sentence.
I was playing with the idea that my MC could have an older property that had pre-existing tunnel system, perhaps used for rum-running in the roaring 1920's or escaping slaves as part of the underground railroad and they (or their company) could own the place and the buildings, rent them out to drug smugglers, human traffickers (snakeheads/coyotes) and thus be valuable to those criminals so they'd kept their mouths shut about the "secret" route. My big challenge is...if this area is such a valuable route, what keeps the drug smugglers from seizing it?
Law enforcement....sad as it is to say, is probably not going to work too hard on stopping the flow of drugs or cheap labour into a region especially if various street gangs are battling it out for drug dealing turf - visible dead bodies, public shoot-outs will drain police resources and upset the voters. If the property is set in a region that has a huge geographical area and is sparsely populated (low tax revenue = low public services like police) plus has a large transit network of semi-trucks driving through on the highways and roads......it's easy to stay under the radar.
I think the real problem for "successful" criminal is when they flaunt their wealth or power. There are probably tons of criminals that have $1 million, $5 million and just carry on under the radar. They probably have more to fear from their own henchmen and criminal allies than they do from law enforcement. It's when they get up to the $10, $20, higher millions of wealth and they want to enjoy their life that it becomes a struggle.
And, some crimes get more attention than others. Crimes against children or the elderly or law enforcement or other "worthy" victims get more attention than crimes committed against the homeless or addicts or transient populations like travelling salesmen or hitchhikers.