My best guess is the weather and watering. Peppers are heat lovers and cool night temps or cool soil can influence the taste of peppers and tomatoes. The blossom end rot tells me they may also have suffered from irregular watering. Letting the soil become too dry and then adding too much water at once will cause the blossom end of tomatoes and peppers to rot. The spider mites often cause problems when the plants are stressed from lack of water. I do not think the neem oil had anything to do with the flavor of the peppers. Next year, wait a bit longer to put out the plants so the soil will be warmer—mid June in many areas. Then use a mulch on top of the soil to seal in moisture, but check the soil often by poking a finger down at about 3 inches. Keep soil evenly moist but not wet. You'll be picking a peck of perfect peppers. Keep growing, Marianne Binetti