Dental fricatives are somewhat 'poor' sounds generally - they don't have a lot energy, so they tend to be difficult to recover in noisy environments. And note t

Dental fricatives are somewhat 'poor' sounds generally - they don't have a lot energy, so they tend to be difficult to recover in noisy environments. And note that even in English dialects these dental fricatives have a tendency to shift to other sounds (e.g. to alveolar stops). While English pronunciation/inventory may be harder than say Spanish or Italian, I think it's pretty middle-of-the-road compared to other languages in terms of size of inventory/difficulty.