My personal regular experience is more positive. Perhaps I have been lucky in choice of locations, but most weeks working away from home I have found my Three phone sim hotspot more reliable for BBC iPlayer and Netflix use most evenings than the WiFi in many hotels (and for a while, better than a heavily congested Virgin Media service in a house I lodged in).
Often, my work laptop is connected and using a VPN to some office network, and a personal VPN is operating on my tablet so I can see Netflix content from another country.
There have been times when I have had poor signals, but not so often (other than really too deep within a building) but whenever I have had a good signal, my streaming has been largely uninterrupted. It has been a while since I have used a Three MiFi, perhaps it is unfair comparing older technology with a 4G smartphone (Samsung Galaxy Note 3).
Until June last year, I had a corporate O2 service with streaming, and found better coverage overall for phone calls but much poorer data services. Not tried EE or Vodafone.
As an aside, I am now working in Helsinki every other week or so, and have installed a dual sim adapter in my phone and switch to a DNA sim when in Finland which turns in a excellent performance (easily >20Mbits most of time, 17 Euros a month unlimited inc tethering).
YMMV.