They don't like it because they are grumpy and think it looks bad.
The default icons are all small, unbold, and do not make use of color. The icon mod inverts this so that all icons are large, bold, and multicolored. Both extremes are ineffective for actually providing you with information.
Ideally, icons would be sized/bolded according to their tech level, and only important things would make use of multiple colors. This is why I just made my own icons where T2 units are 1 pixel larger than T1 and T3 units are bolder than T2, then it is simple to tell them apart in mixed armies while still remaining uncluttered. All the structures remain default except the ones that are important to notice.