There are actually two sides to this. The Generac will indeed produce a good "true sine" and as long as the unit is in good working order and not over loaded, it will not harm your electronics. And yes "square wave" or " modified wave" is not good for most electronics as it "simulates" the wave pattern in a step fashion and is considered to be "dirty power".
But, if you have a "true sine" inverter type generator, it will put out the same quality power as the grid will.
The biggest difference between a "regular" generator and
a "inverter" type are the electronics in the "inverter" type generator.
The built in electronics will compensate for over loads and hard starts without compromising the voltage or
Hz level. If either of these get pushed to low from a large load on a regular generator from being bogged down on a hard load or start it could damage your sensitive electronics. As a inverter type will simply shut down the AC feed like a switch so as not to damage your electronics.
I personally agree with the first poster in that you don't really need a "inverter type" generator unless noise is a big
issue. The other benefit of a "inverter type" generator is the fuel savings. Regular generators cannot compete in these two fields. But that being said, "inverter type" generators cost a lot more than a regular gen set and most do not go above 3000 watts unless you tie two together. And that more than doubles a very expensive gen set setup.
And that of course smokes your gas savings right there.
A good generator with an electronic voltage regulator will do the job nicely as long as you measure your loads
before purchasing and make sure the generator is big enough to not only handle your loads but also handle the
starting surges without bogging it down to a dangerous level.
I have a Generac Primepact 66Lp "liquid propane" generator and it produces perfect 120 across both legs and provides
a good sine wave for all my electronic needs. It was originally made for motor home use but has been converted for my backup. Generac has a good name for it's self and for many good reasons.
If your looking for a good gen set then look for one that has an oil filter and pump system. They are the best and are built like a regular car or truck motor in it's lube set up. The cheaper gen sets have neither of these and use a "splash"
oil system that uses the crank case to dip and splash oil into the rest of the motor. While effective, it's much less durable and as such, is used in lower quality motors because they are cheaper to build.
Also, if you can find one, a lower rpm gen set (2800rpm) vs. (3600rpm) generator will not only last longer but are much quieter than the 3600 rpm versions. They are more expensive usually, but if found used and in good condition are well worth the money. Mine actually runs the motor at a lower rpm and then through a pulley belt system up's the rpm to 3600 on the stator side. This is a very quiet set up and is considerably quieter than my older portable 5500 watt Generac I had a few years ago. It also stops all the vibrations of the motor from traveling through the stator side and is bound to help keep the bearing from wearing out so fast.
One more thing to look for if searching for a gen set is to find one with more than one cylinder. Over head valves are pretty much the norm now but having two cylinders will lessen the load on the motor and they are built stronger.