Is one hour 40 minutes layover in Frankfurt enough?

Experiences in FRA vary quite a bit. I've been through there when things were pretty quick and efficient. I've also connected there when things were slow, chaotic, and shockingly dysfunctional. It's not high on my list of favorite connecting airports - it's huge and there are a LOT of flights that go through there - but it's often "on my way".

If you're looking at a potentially tight connection, you need to look at what happens if your original connection fails. Assuming you're booked on a single ticket, how bad would the consequences be if you misconnect? The airline would put you on the next available flight...is that 90 minutes later, or the next day? FRA is a major hub for Lufthansa and United (partners) so there's a ton of flights that might get you home the same day if needed (though maybe with an extra stop or two). Take a look at what your options would be.

That said, I've made it on every connection in FRA I've ever had) though a couple times just barely. I've also sat and waited in the airport for many hours (when I was on two separate tickets and booked a long layover for peace of mind - I've got a 5-hour layover coming up in September on separate tickets). I'd prefer to connect there on my outbound travel rather on my return, but will do it as needed.

Frankfurt certainly isn’t a favorite place of mine to transfer but I have always made my flights.
In one case, they successfully led everyone lined up for our flight through the airport to a different gate.

We have traveled to/from SFO to Italy many times going through Frankfurt, and have never missed a connection (knock on wood). Neither has our checked luggage, though we often just do carryon.

A lot of changing in Frankfurt depends on whether you are changing airlines and concourses. Lufthansa and the Star Alliance airlines use Concourse B and A/Z (A and Z are in the same building, just on different levels, A for in-Schengen flights, Z for out-of-Schengen flights). Changing between A and Z is just a matter of going up/down escalators to a different floor (and going through immigration/emigration, passport check). That shouldn't take long.

Even if one flight uses A/Z and the other uses B, there is a tunnel between the buildings. It shouldn't take an hour between gates.

Now, if your flights are on different tickets and your luggage is not checked through, you would have to go out of security, claim your luggage, recheck it, and go back through security, then all bets are off. Or if, for instance you fly in on Lufthansa and are flying out to SFO on American or Delta, you will have a longer change of terminal and have to use the SkyLine shuttle. That could be a problem in 1H40.

We flew into Frankfurt 11 months ago to make connections, and our airline didn't promptly check in travelers. We got into Dulles late and all the flights for the next 2 days were full. We had to spend the night and then take a flight into another city. We rented a car and drove home 200 miles arriving 24 hours late.

There are many uncontrollables in the travel world.

I would prefer taking KLM into Amsterdam and then a non-stop to San Francisco. I've been able to make relatively quick connections there over the years.