There are some caveats. Video streaming is typically at 480p resolution (not HD), and T-Mobile may throttle your speeds if you use more than 50GB a month (which is a lot of data to use). Tethering is also at a max of 3G speeds instead of 4G/LTE.
The $50/month unlimited plan is a $10 discount from T-Mobile's regular prepaid unlimited plan ($60/month for T-Mobile Essentials Prepaid).
Verizon's recently discounted prepaid unlimited plan costs $65/month, while AT&T sells its prepaid unlimited plans for $65/month or $45/month. In comparison, T-Mobile's $50/month plan is priced competitively, but there's also fine print to consider with each plan (like throttling, hotspot limitations and so on).