I have a set weekday where I skip two meals and drink water. A few days of simple vegetables and water (a Daniel fast) also lets me stay functional at work. Because that rhythm exists, I can fast on short notice when something is pressing.
The cons showed up when I ignored my limits. A fixed day turns into clock-watching if I’m not actually praying. Fasting while sick or on medication made me irritable instead of attentive. When I only did it whenever my church happened to mention it, I never did it at all.
Christian fasting is voluntarily going without food for a stretch, sometimes on a schedule and sometimes when a need is on my heart. Believers should do it, not as one rigid program.