Except that you need to maintain load on the generators. Load them too light, and they speed up and won’t keep in phase. (This is actually similar to a nice SCADA based attack on physical infrastructure; disconnect the generator for a short while, let it speed up out of phase, connect it back; the mismatch will cause a major current spike and a mechanical shock. Repeat until damage occurs. If you are fast enough, you can switch the thing off and on without the protective systems noticing.)
(Actually, the rest of the grid will keep them synchronized, but then the whole grid can show tendency to drift up with frequency.)
A lot can go wrong if you screw up with the grid load/supply.
Not in principle. Yes in scale.
