.tk domains have a bad reputation because this TLD is used a lot by spammers. The .TK top level domain is ranked second by spamhaus Spammers TLDs ranking.
We found that it cost something close to $22 to buy a .tk domain at namecheap. Taking into account the reputation of .tk, I think it is overpriced for this domain extension. There are several 2 char domain TLDs with a better reputation and equal price or even cheaper. It is the case for .cc domains, .cx domains, .co domains, bz. domains which are not members of the most abused TLDs ranking from Spamhaus.
I think that buying a .tk domain could be a problem with some servers. .tk domains could be a server domain, an email address domain.