Tether concealed its relationship to Bitfinex, which is generally not a good sign. Their auditor also released a statement which, I am told, is auditor-speak for "they failed the audit."
If they are backed 1:1, it requires them to have received large dollops of USD at a time when many financial institutions were wary of giving cash to cryptocurrencies. Without more details on how that USD came about, there is reason to be skeptical of that claim.
