Main strengths: High area control, Good DPS, Great Ult, Easy to position.
Main weaknesses: Projectile shooting, Can be flanked, Low 1v1 kill potential, Low mobility, Low HP pool.
Favourtable maps and modes: Best on maps with choke points as defender (mostly Capture maps)
Unfavourable maps and modes: Not optimal on Control maps and on Payload maps (once the payload is moving)
Defend / Attack specifics (optional): Avoid using him as Attacker (very rarely a good pick). As defender Junk is great to solo hold a single narrow path (Hanamura gate, Temple of Anubis gate etc.)
Good against: Torbjorn, Bastion, Reinhardt, Mercy, Symmetra, Ana.
Countered by: Genji, Pharah, Widowmaker, Hanzo, Soldier, Zarya.