Can big drop-offs and rocky descents be handled by 29er with 120mm
A 120mm 29er with Trail geometry would allow riding down large drops and rock descents and the shorter travel will also allow for sharper turning radius than a longer travel bike.

There are a number of 120mm 29" bikes have been designed in the last few years to have a trail orientated geometry that is more progressive than that of 29" XC bikes (such as the Whyte T129 and the Specialized Camber EVO). They are designed for similar riding that you'd expect on 140mm/150mm 26". 29ers do not need as much travel as the wheels allow for a lot more compliance over rocks and roots. The ride won't be twitchy like an XC bike but it will be closer to the feel of a hard tail than an all mountain bike.

29" wheels may suffer more with more aggressive riding but this has to be balanced with the other advantages of wagon wheels. If stock wheels need replacing you can get a sturdier set to suit your riding.

I wouldn't recommend a 29" if you were expecting to do any big jumps.