In short, they're saying that you're not giving it a fair reading and thus your opinion is unfounded and invalid.
They might be saying that... but they would be unreasonable doing so - under any normal and practical circumstance.
Because:
I teach television studies, so I think I can explain why they say that. What you're doing (reading a dozen comics of more than 640) would be the equivalent of walking out on a film after a few scenes or of dropping a book after 10-15 pages. Forming such an extreme judgment after experiencing something like 2 percent of the work can only be justified if the work is spectacularly bad or offensive.
And how long does it take a film to show "a few scenes"? And how long does it take to read "10-15 pages"? Exactly - not much.
Now, let's compare that to reading, what, 200-300 strips? Hmmmm... doesn't sound like it's worth going through all that if the strip hasn't caught you
long before spending all that time reaching that point.
Especially for the casual consumer of entertainment (and that's all this is).
In addition, long form serial narrative works like comics or like television programs base many of their effects on building up slowly over time or depend on character developments that are foreshadowed in small and large ways over years. To sample a long form serial narrative is to basically deny the validity of long form serial narratives.
And really - there's nothing inherently wrong with that
if there are other factors involved (you know, like people having
lives and other limits on that oh-so limited resource called 'time'). To ignore the time factor is, AFAIC, unreasonable.
It could be, in fact, that if something is designed as a 'long form serial narrative', and it doesn't catch someone fairly quickly (thus forcing one to spend an inordinate length of time evaluating it), then that in itself could easily be seen as a legitimate criticism.
Edit: And, yes, this stance makes evaluating television programs (which can be more than 100 hours in length) very laborious.
Indeed it does - but I suspect those who follow your specific criteria are
extremely few and far between. I'd go so far as to say your vocation provides further evidence of your relative uniqueness in how you, personally, evaluate 'long form serial narratives'.
I'm willing to give JeffB the benefit of the doubt and say that yeah, he probably spent "enough" time to determine - for himself - whether he gave it a fair shake or not.