When trying to take off using Elytra from ground level, the double tap on jump (puts you into glide mode), but there is insufficient time to tap&hold for the firework to activate and provide the flight boost.  Using windows 10 and a keyboard, this is possible and you see pretty much every Youtuber doing it, but users on Bedrock with a touchscreen can't do it.

This could be resolved by providing an on screen launch button that combines double jump and firework activation, or by activating fireworks quicker or perhaps by providing a method of scripting firework activation in a behaviour pack or client.js script (rules would be - if player double jumps (starts to glide) and can fly=true, is holding firework in right hand, was not previously gliding or flying, then activate firework)