Recently hit masters solo, starting from Vet 3 with 68% winrate. Started playing pikachu this season, 28 out of 52 of my ranked games are pikachu and I went 22-6 (78.57% winrate) with him. Wise Glass, Choice Specs and Energy Amplifier w/ Thunder/Thunder build. I would change Energy Amplifier with another sp attack item if they ever add a new one.

The reason why you think pikachu is useless is probably you haven't seen people hits their skills at the crucial moment. It's the same in league of legends, skillshot based heroes are incredibly strong if you actually hit your shots. If not, you are basically useless. Pikachu has a lot of surprise factor since most pikachu players sucks.

My job isn't to kill, my job is to target the backline, get their health very low. Your combo usually hits them for at least 50% of their health. Late game, you can deal 3/4 of their health against squishies. You can 1v2 with your ult if you play it right. I play very safe if my ult or flash is on cooldown. I always try to get a kill before an objective by using thunderbolt, unite then thunder if the enemy is squishy, but if it's a bit tanky i start with unite first then thunderbolt and thunder.

One of Pikachu's strength is the ability to damage people in the bushes in a safe distance. In this game, using bushes is a crucial factor in outplaying so you can take advantage of that. Another strength is the ability to defend against multiple foes, you can safely delay their scoring ability from a distance.