What are the effects of having flat feet?

It is more about the: Stability of the feet than whether the arch is high or low. A low arched but stable foot is perfectly fine. An unstable arch is one that flattens out too much when bearing weight. The instability can lead to a number of problems such as bunions, hammertoes, plantar fasciitis/heel spur, achilles tendonitis and more.

Flat vs pronation : Flat feet are a structural problem which may not always be bad. Pronation is motion in the foot that allows the foot to adapt to uneven surfaces. Over pronation are excessive pronation causes the ligaments of the foot to relax to long allow strain on ligaments and foot deformities, such as bunions hammer toes neuromas joint pain and arthritis.