Using Sama's definition of a hard-tech product or business as "where there is doubt that the technology can be built at all", a bunch of successes come to mind.
1. Tesla (Market Cap ~$29B w/$14B+ in pre-orders)
2. SpaceX (Valued north of $10B)
Elon's companies faced massive doubts about tech feasibility, both when started and at basically every-point since.
3. Oculus.
Sold for $2B in March 2014, including $1.6B in FB stock that's now worth 2x more.
4. Machine Zone
The messaging platform for Game of War/Mobile Strike is very "hard tech". (Google/AWS still lack equivalents and MZ was kinda forced to take a "time out" to develop it.) In 2014 the WSJ said they where fundraising at close to $3B and now their cash flow powers global TV add campaigns.
5. Cruise. "Hard Tech" and a big win for YC.
6. Zoox. They started w/a prototype and have made huge progress.
Many new business lines at public companies faced huge doubts about tech feasibility and are now clear home runs:
7. Nvidia
Every new chip is "hard tech" and they've grown massively in the last decade.
8. iPhone/Apple + Android Inc.
Smartphones are really, really, "hard tech". So many doubts on both the tech and product side are now forgotten!
9. Google (mentioned by others)
In addition to search/adds/Android, I'd say maps/gmail/infrastructure/android are all examples of "hard tech". Each was viewed as "hard tech" when released. (Contrast Google Maps w/Apple's disastrous maps launch years latter.)
10. AWS
If you had pitched AWS as a startup, in late 2003, you would have faced lots and lots of technology skepticism. A phenomenal success.
11. Amazon Echo
Hard tech, took a massive team to develop. Doing great as a product.
Crossing "the valley of death" in commercializing new tech is really hard. But there are vast success!