See title:
works fine:

title @a actionbar "hi"
title @a actionbar {"score":{"name":"@s","objective":"FlyTime"},"color":"green"}

Kicks clients:

title @a actionbar {"translate":"%s","with":[{"score":{"name":"@s","objective":"FlyTime"},"color":"green"}]}

errors:

[20:22:39] [Netty Server IO #1/ERROR]: io.netty.handler.codec.EncoderException: String too big (was 34931 bytes encoded, max 32767)
[20:22:39] [Server thread/INFO]: FVbico lost connection: Disconnected

it doesn't crash when you just run it once. It starts to freeze (and eventually) crash if the command is run often.

• Try putting the function in a 'needs redstone' repeating command block with a comparator attached to it which reads the output of a hopper chain.
• Then throw an item in the hopper chain (so the comparator turns the command block on for a few ticks),
• you'll notice the fps gets less and less,
• and eventually it's crash (if you do this twice in a row, for example)