Three-time Formula 1 champion Stewart says the potential for Mercedes to abruptly withdraw from the sport if it underperformed would unnerve him in Hamilton's position.
"I think Ross Brawn is a wonderful man. I think Daimler Benz is one of the greatest companies in the history of motorsport," said Stewart on the AUTOSPORT Stage with KX at AUTOSPORT International.
"But they have to win. The Silver Arrow has to win.
"Whether it was Stirling Moss and [Juan Manuel] Fangio, whether it was [Rudolf] Caracciola and the other great drivers of the thirties, when Mercedes were in the sport they had to win.
"And if they don't win, they get out.
"That's the risk. If for whatever reason, they do not win, it's a five-minute board meeting decision with a bunch of people on the board who are not passionate about motorsport.
"They like the results if they win. If they don't win, it's yesterday. And that takes five minutes.
"If you were looking at it in a conservative fashion, you'd say 'I've got to stick with McLaren because I know they're always going to be racers'. They will never be out of motor racing."
Stewart also believes Hamilton's inconsistency remains a stumbling block for the 2008 F1 world champion.
"When he's on form, he is the quickest," said the legendary Scot. "But [his ability] to do it consistently is not yet there."