1 Reason: Unless you already love that brand unconditionally (e.g. Apple), and don’t mind being seen as biased with valid reasons supporting your arguments, being exclusive (or an Ambassador) of a product or company might mean loss of engagement from competing companies and products which in turn will narrow your perspective of understanding what benefits the consumers, rather than the companies.
Similarly, being exclusive to a brand usually will tie your hands down to only singing praises and limited/narrow view of the disadvantages or improvements that product you are representing has or needs. This just restricts your editorial voice, unless you are already convinced they are and will always the best.
So next time if anyone asks you to be exclusive to their brand/company/product, please consider wisely and don’t limit yourself to a particular technology, unless you believe in it 100% through and through.