Jack Flag is an effective Ally that will struggle to find a place in many Hero's decks.
Jack doesn't have any cool effects on entry, doesn't provide any keywords, he just provides a solid 2 THW or 2 damage for four turns, and then you chump block with him.
Although he only has 3 life, he actually is a bit above curve. His Hero Action doesn't require him to take consequential damage, and his Ammo Counters can be "saved up" if you need to Thwart a couple times in a row.
He benefits doubly from extra life and healing, as he gets two activations for each point instead of one.
Having a Leadership teammate would greatly increase Jack Flag's impact and sustainability. Comms Implant was built for him, bringing him to 3 THW and granting an extra life. An Avenger Hero can use Inspiring Presence to heal and ready him, accelerating his tempo. Using Ready effects (Command Team, Get Ready) help him with getting his value faster.
He costs four. Four cost Allies must be carefully chosen and in limited numbers.
Jack is slow. He can take four or five turns to get his full value, which can be too long in some circumstances.
Although, he is a Guardian, Jack doesn't really care for Blaze of Glory, as his hit points are precious to him.
I like Jack, but his inclusion must be thought out as to why you are including him. He provides good Thwarting...in Justice, where you likely have a number of cards to remove Threat efficiently. He adds some damage, but it's slow without extra support, and you can't use attachments to increase his damage since its tied to a Hero Action.
I rate Jack Flag a B: Effective in most decks, and can truly shine with a little extra support.