Le Mans? Weak. Turbo pumps on rocket boosters probably operate in the highest stress environment of any engine ever designed! Extremely cold fuel pumped to extremely hot engines at 10+ G's at microsecond precision baby!
The point is, just because the WEC and Le Mans (my favorite race fwiw) is harder doesn't mean NASCAR engines don't also deal with a ton of stress requiring advanced development. Also, there is nothing in the sporting regulations (http://www.24h-lemans.com/wpphpFichiers/1/1/ressources/Pdf/2015/24-heures-du-mans/regulations/2015-fia-wec-sporting-regulations.pdf) about not fixing engines. I've seen plenty of radiator replacements, maybe a turbo or two, and a full rear end swap (transaxle & suspension...in 4 minutes!!!). All it has are time penalties for using more than one engine for qualifying and race.