I’m not sure if this is intentional game design or a unique experience to me, but it seems like the effect of Deep Freeze (its immunity, primarily) is less immediately responsive compared to say Flame Shield, due to its ‘casting animation’. Maybe it is designed this way as to differentiate it as an ultimate ability rather than a 5th defensive spell, but it’s pretty damn frustrating hitting my deep freeze to avoid a dangerous mechanic, just for me to die while my character goes through her animation to cast it.
It can be frustrating for sure. To me it seems like it was designed this way on purpose, as it might be too OP if it were instant. It gives you unstoppable plus 4 seconds of invulnerable, so it’s already a very clutch ability when you’re in trouble and/or CCd. Having to time it a split second in advanced seems appropriate for balancing it.