Why are there some claims the HACman mixture control device is problematic/bad?

No answer for you, just speculation - 

Having read a bit about this aftermarket gizmo for Bing type carburettors, I wonder about the placement of the,  to atmosphere, vent on the firewall.

I have a few years experience in Rotax 912 ULS - one point is often repeated/emphasised - the float bowl breather tube must exit to an area of the same atmospheric pressure as the carburettor air intake. Failure to observe this instruction may cause any number of carburettor issues.

It may be, that on some aircraft, the location of the HACman air control/vent on the firewall results in a different pressure impacting on the float bowl, compared with the carburettor air intake, resulting in mixture problems.

On other installations, the air pressures may be the same and the HACman may deliver the hoped for control/improvements to carburettor performance at different altitudes.

Is it still available for sale anywhere?

From the many Hacman owners I have talked to over the many years most have removed them. You don't really need it and it's better to leave your engine as is. Remember there are reasons Rotax does things and it's usually better to leave things as is or pay a hefty price later.

@Sean, thanks, that's a neat viewpoint.

@Mike, I can't really say. I haven't looked at it, but I was wondering what about the principle is risky.

@Roger, thanks for chiming in. There's something about what you say which doesn't make sense to me, though. I don't know many airplane owners who would remove something which was installed unless there was something they specifically didn't like. Did those owners give any indication about why they removed it? (I also don't know that it makes sense to say that we should always leave everything alone and not question if we know how, in the 2020s, to do things better than in the 1980s.)