The problem with "you're smart" is that it does not reward you for the habits that helps you improve and continue to achieve that state, but for an achievement which may very well have involved little effort on your behalf.
If you are currently coasting when you are given that praise, then the implicit lesson is that it's ok not to work hard. It comes back to bite you once you're eventually hitting real challenges.
If you praise someone for being smart after they've actually put in lots of hard work, perhaps it'd work better. But praising the effort itself is "safer" because it works in either case.
Also, to the extent this functions as a "self fulfilling prophecy" it is still better to get people to work hard, because you can always work harder, but it's not at all obvious that a kid will make the connection between praise for "being smart" to putting in more effort to continue getting that praise.
For my part, I feel that the fact I was regularly praised and rewarded for my achievements as a kid, rather than for effort, had a bad effect on my study habits that took me years to undo once I started being challenged. All through school I basically got reinforced that I should expect great results and praise for pretty much no effort, so I got used to slacking off. I rarely revised for tests more than an hour or two. I expected to be finished with a test in less than 1/5 of the allotted time. Because that was just how things were, and I knew I'd do well enough to be praised for it. It was all very well meaning, to be sure, and I don't blame anyone but myself, but then I started university, and basically had to learn to study from scratch because I couldn't coast through every course any more, and it was a nightmare.