"I've been doing keyword research for years, and I can tell you that dealing with hundreds of keywords and dozens of spreadsheets is enough to make anyone lose the will to live. Keyword Researcher is great because not only does it let you easily find keywords that people are actually searching for, but it has a clean and simple interface that makes it easy to keep track of what you've found. If I'm stuck and need some new ideas, I just fire it up and usually find several things to write about in a few minutes."

Note: If you can’t justify using Ahrefs on a monthly basis, you could sign up for a month, do your KW research and cancel. That said, if you can justify the monthly pricing it’s well worth keeping because you’ll get access to the ongoing functionality such as rank tracking and web monitoring. It also means there’s no need to use any other tools to track rankings or monitor mentions on the web.
Google prefers to not share long tail searches in this tool – because the tool is designed for Google Adwords advertisers. If advertisers bid on those long tails – they would have less competition and therefore pay less for the Adword, and therefore Google would make less money e.g. bidding on the term weight loss for girls under 18 is cheaper than weight loss. 
Answer: Google Keyword Planner was created to find keywords for Google ads. It will likely not produce very good keyword suggestions for SEO or content marketing purposes. Google Keyword Planner often hides good long tail keywords. To find long tail keywords for SEO, you can use Keyword Planner alternatives such as Google Trends, Google Suggest or Keyword Tool.
The limit on manual keywords could be higher.  I personally wish we could input 10,000 keywords at a time, instead of 200.  However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input.  Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).