Are you saying that the LED on the inside of an opto coupler will blow before the transistor on the output side is fully open?
What do you mean with "fully open"? In every transistor, the output current is limited by the input current.
what does CTR stand for?
Current Transfer Ratio. See any optocoupler datasheet; it's essentially the same as a transistor's h
FE, but measured in percent because the number would be ridculously small otherwise. For many common optocouplers, the guaranteed minimum CTR is less than 100 %.
You can blow a 'normal' transistor by sending too much current either through the base, or through the collector. With an optocoupler, it's unlikely that the second case happens because you will not be able to get enough photocurrent into the base.