If the agency’s reliability varies by product, I’d consider them generally not reliable, unless the problematic cases are because of a specific kula that they rely on.
Yes, but it may also depend on what you rely on them for.
For example, I may rely on a particular hechsher for michig, but not fleishig. I might not be comfortable with a particular hechsher for cheese production, but use a candy with their symbol. I may use a hechsher for packaged goods, but not food service (restaurant/catering). I may not accept what they say about other agencies, or assumptions they make about what does not require hechsher.
It takes some research to make some of these distinctions, so some take an all or nothing approach.