Stablecoins are not bad to invest in, moreover it's not like you are investing in it because there is no profit to be gotten out of it rather it is more like using stablecoins to protect your funds during bear market which you can still convert back to your favourite coin when the market changes for good.
However, about Tether and Libra, both are two different projects or platforms because one (Tether) is already developed for years and currently can be seen as the best stablecoin with huge volume etc while Libra is yet to be launched thus no one knows actually what to expect from it. So therefore, in a bid to answer the question, Tether is worth the attention given to it.
Stable coins are not a bad investment? Lol, you don't make any profit off it, so there is nothing to invest on it and you know what. But, it is still a bad investment. As far as I know, your tether tokens can be frozen anytime. I am not sure, but I think if you come across some stolen tokens, they can be freezed.
Yeah, tether and Libra are different things. Is Libra still under development? I though it already died before it even got to launch.