Maybe I am just being stupid, but Battlegrounds really lose a lot of units and gain little except for the lucky jackpot. This is normal?

There's nothing extraordinary in your situation. As you know, some Units are more effective against particular Unit types than others. Choose them wisely.

Feed Small units in for ale effecincies

70k attack vs 28k defence? Level 68 Battleground

http://prntscr.com/bsyqfx

Would have been better if you had send in a screenshot including  your "Used troops" and the and the Troops defending the BG.