Custom elements can look amazing, but they tie you to a specific solution. A wildly unusual tap or a one-off shower screen size might be hard to replace if it fails. Special-size tiles or boards can be difficult to match in future repairs.
Standard-sized fittings are easier to source, easier to replace, and usually cheaper. If a mixer breaks, you can find another brand that fits the same holes. If a tile cracks, a close match is more likely. Hardware like hinges, drawer runners and handles in standard sizes also give you flexibility later.
You can still get a stylish look with standard components by the way you combine finishes and shapes. Going too custom everywhere might impress today, but often becomes a headache five years down the line.