As i have said, i would not alter this ability for my games.
But if it is needed or preferred to change it so there is a chance, i would suggest.
"Reliable talent does not apply when the check is at disadvantage."
That allows for unfavorable circumstance to create the chance of failure and the master of skills only is safe under "normal" or "favorable" conditions.
That would be kicking the rogue while he's down! The time that the ability would be at it's most beneficial and useful - houseruling that it doesn't apply. And frankly, that's exactly what this ability is supposed to guard against - maybe another class is under stress and would muck it up - but the rogue - he's got this.
Or if you want even more...
"Reliable talent ONLY applies on checks rolled with Advantage" but to me this makes it fairly weak, very weak.
As you say, this would make the ability just about useless.
5e doesn't (as far as I'm aware) explicitly state tiers of play (like 4e does) BUT they are certainly still there. 11th level seems to represent that tier changer for most classes. Many classes get some beefy "you're no longer a mere peon" abilities at 11th:
Fighters get a 3rd attack, Paladins get an extra 1d8 to each attack, Barbarians get relentless rage (the "I'm not dropping" ability), mages and clerics get 6th level spells, etc.
Reliable talent is the rogue's tier changing ability.