True online ups systems provide complete protection with very few drawbacks. There are three main reasons we prefer true online versus standby and line-interactive.
Incompatibility between a uninterruptible power supply and generator is a common problem that is often only discovered at the worst time - when the power goes out. What often occurs is an oscillation where the ups will accept generator power briefly, but will then switch to battery when the generator frequency or voltage moves out of acceptable range.