Haha Estonian is far harder than Slavic -- it's Fenno-Ugric.
It's more about the lack of good materials to learn than the language being hard itself.
(All of the following comes from the assumption that Estonian is very similar to Finnish)
This is only partly true.
I'm currently learning Finnish. Well, it's resting a bit at the moment, as moving there takes up most of my time right now, but I did learn for about a year and will continue with it after September. Probably for many, many years.
It is true that learning Finnish is party hard because of a lack of good materials. All those (pretty good!) websites with small lectures, masses of exercises, subtitled movies, games, etc.
Everything you usually use to learn a language such as English, French, German, Spanish (? Not sure, this one's a guess) simply by consuming stuff in it that are of your interest anyway.
It doesn't exist for Finnish. And if it doesn't exist for big brother Suomi, it won't for little brother Eesti.
It's basically down to less than a couple of grammar books. And if you are capable of learning a language by just reading a grammar book, you have my respect. I know I can't.
So... you basically have to come up with your own training program, get the help of some natives, etc.
Yeah, the lack of material makes it harder.
BUT...
The mere fact that Finnish (and thus Estonian) has absolutely nothing in common with any other language (except Estonian, maybe Hungarian) is what lifts the difficulty more significantly than lack of good material.
When you know a language that is partly based on / influenced by any Germanic / Latin language (as most western languages are), learning another from that family is much easier as you already know quite a lot from your own language. Words are often similar, as are grammatical rules, etc.
Basically, you learn the language starting from 0 here. As a baby would. Just that your brain is not empty any more - and thus much harder to fill with new things.
Oh, and if Estonian is as similar to Finnish as you always hear, it is a horrible mess of an extremely large number of grammar rules (and sub rules, and exceptions from sub rules) and is in dire need of a language reformation (like German had multiple times in the last 200 years).
Bottom line, if you are not crazy about learning a hard language, don't even start.
Oops. Thread derailed.