i'm not calling talonflame bad because i'm not advocating it to drop to tier 4 or something; it's still a solid pokemon that can shine, namely against a team with azu rachi like you mentioned. however, struggling against the majority of common teams is not a trait that a tier 1 'mon should have. it still has its niche: good revenge killer late game, performs well against hyper offense, has speed control, etc. but the niche isn't as needed these days against the omnipresent bulky teams.
yeah definitely i even said you never said it was bad lol
lets look at some of the other tier 1 mons and see if they struggle against common threats/teams
aegislash: fares ok against azurachi, does big damage to rachi and "walls" (not really) azu if its not knock (but almost all are knock). best check to hyper voice and wide guard's lando, and if it gets hit it activates wp and hits hard.
amoonguss: bad against sun, rachi, but great against rain. punishes physical attacks (kang, terrak) with helmet, fares well against diancie and gard (takes hyper voice, spores) beats genies, but gets walled by tran and blown up by zard/talon
zard: loses to genies (scarf rock slide, tw/tbolt from thundy), gets bopped by kang (but ohkos if 252/252 with overheat), and loses to talon. however, beats tran if hp ground and no ancient power (which isn't common), fares rather well against gard, forces the protect on diancie, blows up aegislash and amoonguss, and has a good matchup against rain.
diancie: walled by amoonguss/aegislash, outsped and ohkod by scarfdog, ohkod by offensive thundy w/ flash cannon (which isn't too common) but beside that, does well against everything else beside sun/rain
gard: if you aren't a steel (2 in tier 1) you aren't taken hyper voices too well. provides good spread move for azurachi but walled by rachi. however, due to its low speed + defense, it it vulnerable to kang and lando. probably the worst matchup against top threats beside talon imo.
tran: beats sun, talon, amoonguss, aegislash, walls zard and gard. bad against rain, azurachi (nothing to azu, rachi takes hw) and kod by diancie, low kick kang and lando. ok against thundy
kang: walled by slash, nukes almost everything else. not good vs. azurachi since it can't do anything to rachi. does take +6 aj if you ev enough
lando: best mon tbh, beats slash, zard, diancie, tran, kang, talon, thundy. good aginast sun, loses to rain, fares well agaisnt azurachi
talon: you explained it well, worst matchup against top tier threats
thundy: twaves everything, ohkos diancie if flash cannon, takes lando rock slides and does massive damage back w/ hp ice. can taunt azu and rachi, fares well against rain and sun, but probably has the 3rd worst mathups against gard and thundy.
looking at this, gard and thundy are the two most "controversial" mons in tier 1. and with talon having even worst matchups, its niche isn't as needed as much, idk if it derserves tier 1. still not 100% ready with it moving to tier 1.5