Failing to deliver promised research Will people look down on me if I say that I plan on doing research that I end up not doing due to various reasons, especially if I'm a PhD student?

I guess it's somewhat expected, and that experienced professors don't always believe that I'll end up doing what I say (because unexpected bugs and events happen all the time). And sometimes you also end up going on detours.

That, and professors always say that they do things on smaller timeframes than what it really takes them to do.

I guess if I always meet my commitments, then people might believe me more. But on the other hand, it always helps to get more feedback on ambitious projects that I don't necessarily believe that I will finish.
There is a huge difference between saying that you will do a given research effort, and saying that you will obtain a particular result. If you have guaranteed the latter, you made a mistake, it's research, there is no guarantee on a result !

So, if you give a work planning to your adviser, you have to stick to it. Of course, you can have (once, not twice) a real problem that ruins your effort (house on fire, a relative at the hospital, etc.).

To summarize, you can promise that you will work, but not that it will work ;) If you always fail on your commitments, you will never be seen as reliable, and people won't work with you, it's that simple.