Well, no one product will detect everything all the time.
If you believe that, I have some beach property for sale in Montana.
Oh, and a bridge in New York...
If you found an infection that your favorite AV product is not detecting, submit the infected file or files to the vendor. That is the only way you will get detection sooner rather then later.
If you just complain about the product not detecting an infection and do not submit a sample, well, sorry to say, but you are just part of the problem, not part of the solution.
Sometimes, it is easier to let a infection take place and then remove it.
Some infected files are morphed and are very difficult to detect. But the actual infection might be easier to detect and remove. So yeah, it does happen that the PC gets infected before the infection is removed.
As far as SEP not detecting 'schtuff'...
If you go to other vendor forums, you will see exactly the same 'complaints'. Sometimes it even looks as if the posting was copied and pasted with just a change of the vendor name.
Another thing is that some people use multiple products to detect infections.
Here is an example:
"SEP did not detect this XYZ infection!!, I checked with ABCD and THAT found the infection!!!"
Okay, nothing wrong with that.
But was there a consideration that XYZ was right, and that ABCD was a false positive?
Most times the answer is 'No'.
ANother thing is that some of the log files are misinterpreted.
Tracking cookies will be seen as 'virusses'. Or worse.
As for wondering if you can 'trust the product'.
Considder the source. Who is yelling? What are they screaming about?
If I listen to my neighbor, who has a son, who knows someone at school, who's cousin has a computer that was infected with a virus and that virus was removed by a product that he tried once and trusted, well, I should be listening to my neighbor, right? Or maybe not?
Oh well, just my 3 cents worth (inflation and such).