Semi-valid reason #1 for the hostility might be that it's hard to do correctly. The same's true for wetlands - they could be screwed up by incompetence, bad luck, or corruption. The key is to put in safeguards, though, not to throw out the concept.
Semi-valid reason #2 is that reducing other people's emissions should be secondary to reducing one's own. No kidding. The problem with making this into a club to strike down programs you don't like, though, is it's forgetting that we're just at the beginning of the process of controlling carbon, and baby steps need to be rewarded, not stifled for being anything short of Fred Astaire.
Case in point - Pacific Gas & Electric's Climate Smart program that allows forestry offsets. Read Gristmill's critique by Joseph Romm here, and then PG&E's response here. The additionality problem is solved and the albedo problem is solved. What else have the critics got? At the end, it seems like PG&E has taken Romm apart.
Of course, that's not totally true. The forestry protocols here assume the level of offsets based on comparing emissions to a maximum legal logging rate and maximum development intensity for converting land from forests to residences in California. Both of those protocols involve areas where I have professional expertise, and the assumptions of maximum harm in the absence of an offset will often be wrong. Logging and land development for legal, political, and economic reasons will occur at a much slower rate. In other words, the offset level is exaggerated.
Despite that, it's worth supporting. This is a baby step. Future developments will tighten down the estimate. Most importantly, it does result in reduced emission, just not as much as the protocols say they will. There's no reason to throw this baby out.
UPDATE: I messed up - the albedo discussion is in a later comment, here:
Contrary to what is stated in [Romm's post], the Registry is not oblivious or indifferent to the potential albedo effects of forestry projects, (which are still being understood by the scientific community). The Registry's California Forest Protocols and reduction projects associated with them, however, do not decrease the albedo for the following reasons:
1.) The Conservation Management Protocol relies on implementing sustainable management practices that maximize the sequestration of carbon on land already covered by forests.
2.) The Reforestation Project Protocol relies on reforesting land that has historically been under cover by forest in the first place.3.) The Conservation Project Protocol relies on maintaining existing forest cover, in other words, protecting forests from conversion.