I would say Tether is trustworthy, since I have used it myself and never had any problems with it. However I only use it for trading bitcoin short-term: When bitcoin is high, I trade it to USDT, and when bitcoin is low, I trade USDT to bitcoin.
That being said, I have used it for sending and receiving, and never had a delay, so the problem is probably with the exchange you were using.
1. I'm using Poloniex.
2. I'm 200% sure the tarnsfer has no problem. Because it has been confirmed by 994 blocks by now, if you understand how to use omniexplorer to check the transaction.
3. Tether Support Ticket Opened [#T2888**] last week, no reply by now.
4. I had the same problem for another previous transfer (support ticket [#T7317**]), and their support said "We've checked and your transaction appeared to be stuck in a pending state in the system. We've corrected the issue and you should see it credited to your account properly at this time", it means it stuck in THEIR system! So they could correct it. But does that mean for every transfer I need to raise a support ticket?

5. Please remember the private key of our Tether wallet isn't held by us.