What's the biggest problem with keyword search tools?

Almost all keyword tools have some issues.. that I don't even use them anymore.

Nothing beats doing it manually.

But, is definitely always looking around for something better to come around.

I like KeywordXP which is super fast at finding long tails, but has its own faults in that it doesn't add words before you one you use, just expands on them. But it's a great tool otherwise. Market Samurai is indeed clunky, but I forgive it because when you delve into the parts of it that don't revolve directly around keywords, it's great to see the competition and find useful curation and backlink resources.

Use Google Keyword research tool. I think it is the best.

The main problem for all the Keyword search tools is that you're not guaranteed to rank your page for any keyword. OK you can get ranked for some days but then, nothing...everything is changing.
An other problem is to be "out-dated", I've seen many tools with keywords like "money in 2007" or something like that, they save the same keyword for years while nobody search for it now.

I stopped using keyword tools other than Google's own about 5 or 6 years ago, because they basically all sucked. Too clunky, outdated, inaccurate, all kinds of problems.

I go to the Google keyword tool to get a basic, general list of keyword ideas, and I "log" those into memory for when I'm writing. I update my mental list about two or three times per year for each of my niches. But I then make *zero* effort to follow any "SEO rules" -- I don't write for the search engines, I write for my readers. By being a prolific producer of quality content, SEO naturally happens as a long-term end result.

Plus, I didn't have to deal with any Panda/Penguin or whatever other critter issues most people complain about. In fact, two of my sites went up in ranking because some of the crap clutter above me in the SERPs got cleaned out.

In short, my advice is to have a general idea about what keywords *buyers* are using to search, and then stop there. Write naturally, and for your readers, and write a lot, in a lot of places. The SEO, the traffic, the opt-ins, the purchases...It will all come in due time as long as you do things right and create value.