It depends what the purpose is. I've been on a calorie-in-calorie-out weight loss journey since last August and for most of that time used BMR, as it subtracted a few hundred extra calories each day as a bonus.

For weight loss, eating at BMR is about right generally. And I'd imagine most use cases for calorie tracking apps are about weight loss.

To make it most accurate, I'd personally just add a basic multiplier for what sort of lifestyle people have (sedentary, small exercise like walking the dog, moderate exercise and hard exercise in the multipliers of 1.2x, 1.4x, 1.6x, 1.8x).

I exercise a lot and that typically adds 1000-1500 calories onto my daily BMR of 1500 or so, so the multipliers a lot of BMR / TDEE calculator websites use seem to make sense.

I'm more curious where the food database is sourced from. I tried something similar by getting data from British supermarkets (Sainsbury's, Tescos etc.) and none of them seemed to have APIs, so web scraping was the only possibility.