This raises some interesting questions. Say that an inspecting rabbi sees conditions that are clearly, blatantly unsanitary - but nonetheless kosher. Must he still certify that the meat packing plant in question is kosher?
Also - is this a full-time gig for rabbis, or something that they do in addition to their regular gigs at synagogues? Is it considered prestigious work, or is this where the underachieving rabbis end up?
I don’t think any rabbi is under any obligation to certify something as kosher, even if it is. It’s not like you can walk up to a random rabbi on the street, demand that they stop whatever they’re doing to watch you slaughter a chicken, and then demand that they make a pronouncement as to whether you did so in accordance with Jewish law.
I would expect that there are some that are “safer” and some that are not. I have heard 2nd-hand about a kosher baker who took serious shortcuts with cleanliness because he knew that people who want/need kosher had limited alternatives.
So as in anything, it all depends on how well you trust your butcher/baker/candlestick maker.