Of 123 reviewers, more than half, found the the Harbor Freight separator to be unsatisfactory.
So,.... those folks are all mistaken?
Without going through and reading all the reviews I can't offer an opinion on the validity of their complaints.
Who knows what they are whining about ...
One of the liabilities of relying on others' observations (or opinions) is that you leave yourself open to their own personal shortcomings (inability to observe the obvious, technical incompetence, inability to understand, etc.) Direct, personal observation is a far better route to go, if possible.
What I can say for certain is that I have two of the Harbor Freight automatic drains.
One is somewhere around 5 years old, the other is probably 15 years old ... perhaps more.
Both work reasonably well and do what they are advertised to do.
FWIW, these automatic drain valves are only rated for 100 psi according to HF ... the one on the 30G Coleman runs at 125 psi, the one on the HF 60G runs at 150 psi ... which might account for the fact that they bleed off for a short time before fully sealing after they close. The pressure loss is insignificant ... unless you plan on leaving your compressor on all the time (which is a bad idea IMO)
One of the complaints I have seen is that the plastic tubing gets too hot and melts ... I personally haven't had that experience.
The relevant point is that the valve itself is worth the $10 ... you could throw the rest of the components (tubing, fittings, etc.) in the kit away and replace them with higher quality stuff - like copper or aluminum tubing - and you'd still be money ahead.