Are crunches a waste of time for abdominal strength and are planks good/better?
For abdominal strength, both movements suck.
The crunch is a very isolative movement, it only targets the rectus abdominus (the six-pack muscles) on the surface of the core.
The plank is a useful exercise in the short-term, but it can very easily be mastered and will need to be altered and intensified in the long-term.
If you're going to pick one of the 2, then use the plank, but make sure you do it right. It isn't hard to maintain a plank position, and as a result, you won't get much benefit. However, if you maintain a strong braced core with the rest of your body contracting hard, then you'll actually get some real benefit from it.
Check this video for proper execution: