Well "weakness" in as much as any air force has tankers and early warning aircraft that can be targeted for destruction. I have no doubt they will be well-protected, but certainly they will not be impervious to attack, and somewhere in between "vulnerable" and "impervious" is where the J-20 comes in.
No doubt a tanker or an AWACs might be a tempting target,,,,, but you've got to get closer than you want to, and the US will vigorously defend those assets, so I just don't get all this "hoopla", at least we do have tankers, and we have AWAC's. The J-20's will have to come from somewhere, and they have to "show up somewhere in order to launch weapons at our assets??
Its not a complicated picture if you are on alert, and we really have no idea how effective anyone's stealth is or isn't except our own. With our own 5th gens running a screen, and powerful radars overlapping coverage??? So granted these aircraft are very capable, and possibly L/O, that just makes defense a little tougher!
The J-20 is a land based aircraft, it does not have the mobility of the F-35B or F-35C, or for that matter the F-35A, or F-22 which may meet those tankers anywhere over a vast expanse of territory???