...it makes no sense for the Lightning Mage to be labeled as a Hard Obstacle. It's only got 4 levels of damage, and only deals 1 point of damage to runners.
Conversely, the Aerial Combat Drone obstacle for all intents and purposes SHOULD be a Hard Obstacle. It's got the full 8 damage tracks, and deals 2 points of damage to a runner...
Thank you for asking questions! It's fun to analyze this game even before it's out.
On to your current question: there are a multiple factors that determine an obstacle's difficulty, including health track length, damage dealt, damage difficulty, payout, and bonus ability.
While the Aerial Combat Drone has a long Health Track, and some Damage Difficulty due to a variety of damage types (green, black, and red), it doesn't have large generic damage pools slowing it's defeat (4 or 5 gray for example). You can slowly but surely defeat it as a group. It also has a large (8!) Nuyen Payout and a positive Bonus Ability when defeated (draw and discard).
In contrast, the Lightning Mage may have a shorter and easier Health Track (only gray and blue), but has a much smaller (only 3) Nuyen Payout to compensate. Most importantly, don't discount it's powerful negative Bonus Ability that prevents ALL runners from being healed. The old shadow saying still applies, "Geek the Mage First!"
These factors can combine to create a nearly infinite variety of obstacles, each with a different feel, and can lead to interesting gameplay decisions for the players. The Aerial Combat Drone feels like an agile target, one you have to pepper with shots to eventually bring down. The Lightning Mage feels like a lurking nemesis; hounding you unless dealt with quickly, but giving little reward for your victory. Choosing which to go after first would depend on your current situation, do you need Nuyen from destroying the Drone, or are you on your last legs and need to target the Lightning Mage so you can finally heal. Exciting stuff!