Is it safe to keep USDT amount ~$50K on Tether wallet ?
You have to trust a private company when using Tether. From the
Tether - FAQs:
How do I know my Tether is secure?
Tether is built on top of the revolutionary and cryptographically secure open blockchain technology and adheres to strict security and global government laws and regulations.
This doesn't sound very convincing to me, I've never used Tether. If I have to rely on a third party, I'd just rely on a trusted exchange directly.
Can Tether hold my coins for any reason, or I can withdraw them in any time ?
Withdrawing their made-up token does you no good if they fail to back it up with dollars. Remember when it traded at 91 cents?
Tether Loses Its Dollar Peg:
The price of USDT is currently sitting at $0.91 USD, which has resulted in Bitcoin trading at a premium on some of the exchanges that use USDT like Bitfinex and Poloniex.
I found this in an older thread:
Tether was always sketchy, they are totally transparent yet they don't guarantee anything.
Earlier this year after problems with Bitfinex started 1 tether was traded for 93% of USD value.
If they can create token just like that and won't offer any additional guarantees this crypto is doomed to fail.
To summarize, it's far from safe.