I have not read their financial statements so can't pass judgement on their credit-worthiness. however there was another bank around a year ago that was handing out aa miles and that one got seized by the fdic. texas capital bank (the actual bank you face, bankdirect is just a brand name) is not a household name, and i doubt it would even make page 2 of the wsj if it went under given how many other banks have gone under.

250k gets you 25k miles per month. i doubt whether i would personally go over that limit unless i could seriously afford to lose the money. diversification is the most basic method of asset protection.