You somehow forget that those 5 points of damage you need to destroy Lord of Undvik would otherwise be hurting other enemy units, so Hjalmar gives you a net value of 20 points IF you can destroy Lord of Undvik. For it to work, you need to draw two cards (Hjalmar and the damaging unit), or use your leader Harald, and hope the enemy doesn't buff LoU before you can snipe him. If you can't destroy LoU, Hjalmar is only a 10 point net unit. There are quite a few ways Hjalmar's tempo play can go wrong or can be disrupted. So Hjalmar isn't the problem.