most of the arguments against the A-10 are based on exercises. And indeed, in exercises A-10s get trounced by air defenses and intercepting fighters.
Minor detail. In exercises, A-10s are required to maintain a minimum altitude of ~5k feet and are deemed 'killed' if an enemy air defense platform locks on to them, ignoring the toughness and survivability of the platform. Moreover, at Red Flag, A-10s operating at 5k feet are banned from deploying countermeasures such as flares, as flares could set wildfires.
So yes, when the A-10 is forced to play by exercise rules and compete in the arena that favors air defense and fast-mover fighters, it loses. Big surprise.
Back in the 80's they ran exercises where the A-10 was allowed to play in its preferred environment, down in the weeds so low that the air passing over the wingtips was rustling the grass.
The A-10 was terrifying in those exercises. F-16's couldn't shoot a single one down, because from above the A-10 was too poor of a target even for AIM-9L Sidewinders, so they had to come down in the weeds. There's a manuever called the Warthog Stomp, where the A-10 simply takes advantage of the fact that it can turn far tighter than any other platform when low and slow, and the GAU-8 has effectively zero drop over combat ranges.
Against Shilka's and SAM sites, the A-10 would pop up and hose down the defense site before it could lock on and fire, then drop back down into ground cover before they could effectively react.
The Air Force *hated* the results of these exercises, and imposed massive ROE restrictions on all future exercises in order to eliminate the A-10s strengths and maximize its weaknesses.