Do people not know how to DI out of PK Fire and then shield if it's multiple? You can even reflect it when it pillars to keep Ness out and even trap him in it if you have the tools to do so. (Like Mario's cape)
Ness is crazy strong with a kill throw, spike, uair, bair, fsmash, and even pivot dsmash as somewhat reliable kill options. However, if you're brave enough, gimping Ness is not as difficult as other characters, as his recovery options are very predictable, and he is literally a sitting duck for 3 seconds while controlling his thunder. Be brave and follow him, just be smart too. Remember that you can gimp his Thunder 2 by about 25% by eating it, just remember to tech if you do. I've won most of my Ness battles my caping Thunder 2, attempting to cape Thunder 2 (getting hit instead but his distance wont let him make it) and teching, or sinply FLUDDing him while Thunder is out and making him miss himself.
Basically, Ness is incredibly strong, but he IS beatable.