Joining a well established RP has its perk. 

For one you know it has a solid place, by that I mean it is not in danger of evaporating. 

Next it will also have more substance. An evolving lore, depth of story and perspective. 


But there are also flaws. 

It may have become monotonous. The mechanics may appear overly complicated. The learning curve maybe steep. 

Over all the latter is the problem which most tend to fear, the dread of a steep learning curve may act as a strong deterrence. 


I suppose best solution is to see how engaging the community is. If they are willing to explain said game mechanics, help ease the transition etc. Utilizing these simple factors you can quite easily get a strong footing in what ever RP you are placed in.