If I run my mod (http://cvc.darkstars.co.uk) on Speedmetal most units continuously take damage. Since that doesn't happen in other mods and not to all units I presume it's a bug.
Reconstruction:
1. Start a match on Speedmetal using the CvC mod.
2. Watch Titan health degrade over time.
3. Build factory.
4. Build missile drone and laser drone.
5. Watch the missile drone's health decrease and the drone explode after seconds while the laser drone will remain undamaged.
Only the hovering units seem affected (except for the laser drone which is hovering and unaffected). Random guess would be that the acidic water (does SM have that?) affects units on land if Spring considers them ships or something like that.
On the other hand I think most veterans would appreciate it if this feature was implemented into AA...