If anything Half-Bloods aren't easy enough. As-is, fighting them is more trade-off than gain. You compare them to AI3 in ease of defeat? Yeah, let's go down that comparison:
100 Health vs 200 Health
~20 Electrum vs ~40 Electrum
40 Electrum for EM vs 80 Electrum for EM
1 Quanta from Mark vs 3 Quanta from Mark
Draws 1 Card per turn vs 2 Cards per turn
So in other words... so what if it's the same difficulty as the AI3? It's the same reward, too. In fact, it's a little less reward when you consider that the HB's reward will drop faster than the AI3's reward. If you end the match with 50 Health, it's 15 Electrum vs 27 Electrum. So in other words, unless you can EM the AI5 with 100% accuracy, you're reducing your Electrum per match. In fact, your best-case-scenario against an HB is breaking even with what you would earn against a lower-level opponent. To add insult to injury, the HB's usually have bigger and more varied decks, which means your odds of winning a spin are much lower.
Right now Half Bloods have one thing going for them: A minute, minuscule, slim, long-shot of a chance to win an Upped card that you can either use, or sell for ~1000 Electrum to put towards upgrading a card you want upped. Some say that this narrow chance alone is justification for farming Half Bloods, and maybe it is, but I really don't think that fighting a harder opponent should be such a trade-off.
Anyway, I'm getting a little off-note here. Over-all, I don't find the HBs to be THAT easy compared to AI3. In fact, I can take-down AI3 nearly every time where I sometimes lose to the occasional Half Blood. If Half-Bloods were any harder to fight, they might as well be FGs.