Manga is a bad choice for a beginer because manga does not approach life in a realistic way but rather has its own rules for everything. If you have read a "how to draw manga book" you will notice that the author usually has a section comparing manga with realistic anatomy. Now you may not be drawing realism , like me , you may dont give a shit about it BUT realism is the basis. Before you create a monster you must know how an animal body works for example. If you know the basic body structure of human and animal , you can come up with amazing fictional stuff but , without this knowledge its way too hard trust me. For example , an arm , be it human , werewolf , monstrous , or whatever , must have certain muscles to make it believable , make it look like something that can actually move and function.
The other reason why manga is a bad choice is cause its the easy way. Manga has a certain list of features. Bad guy eyes , cute girl eyes , angry mouth , happy mouth etc. This sounds appealing to someone who hasnt developed his skills yet but its a trap! It leads to stagnacy and you learn nothing from it , just draw the same 3-4 characters again and again with some different base colours and details. No easy way is the right way remember that. You must learn how life works , then you can play with it however you like be it manga , cartoon , realism or any other kind of style.
So , to sum it up , manga is not the devil but if you feel like drawing manga , dont start with it. It will lead you to a dead end. First study how real beings work. Then you can incorporate this knowledge in any style you wish! I hope i helped and im happy to discuss about it further :)