Is it bad to have sharp bends in an internal data cable?
Especially if this was found on the Internet, it would be better if you linked to the source. 

As other pointed out in comments, at high enough frequencies bends in any sort of waveguide, and cables are just waveguides, can cause any sort of troubles (signal reflections, impedance mismatch, unwanted radiation).

All this heavily depends on the geometry and the nature of the medium, and it is impossible to judge the appropriateness of that passage without knowing the exact context.

Anyway the key point is frequency. At low enough frequencies the transmission line effects become negligible, where "enough" means "when the cable length is negligible compared to the wavelength of the signal". In those conditions the situation can be analyzed in terms of stray inductance and capacitance and Kirchhoff's laws.

So, if the frequency is low enough, sharp turns don't matter so much, because they don't influence capacitance and inductance in a relevant way (compared to a smooth bend, that is). 

On the other hand, cable proximity might be an issue even at lowish frequency. A power cable might, through stray capacitance, act as a "bridge" between two parts of a circuit, hence increase interference. But in a digital system this is hardly a concern. If the level of interference would be so big that it consistently swamps the noise margin of a digital line, then probably the system design is seriously botched.

The biggest practical issue in cable arrangement inside PC equipment is usually another one: thermal management. Badly routed cables can impede optimal air flow or thermal conduction paths inside the enclosure and this can really ruin your day.

Electronic circuits usually don't like heat. Once upon a time CPUs and MCUs would die without a notice if overheated. Nowadays probably they would enter some "emergency thermal shutdown" state, stopping whatever they were doing. Read: data loss.

EDIT

searching the Internet it seems that the passage comes from this product rewiev page (cannot link directly). This is a the complete review text (emphasis mine):


  Much cheaper internals than Rosewill's USB2/SATA3gbs model (older one).
  - STUPID connection method to hard drive. EVERYONE that has some savvy in the computer tech world knows, you don't make sharp turns in data cables, or run power cables over data cables. Sharp bends reduce data speed, and power cables over data cables causes "cross talk", causing data integrity issues. This device uses cables instead of headers, and the cables are on the wrong end of the board, causing cables to run from back to front. The internal SATA cable is bent sharply on both ends of the board, with u-turns of 1/2 inch diameter. Poor engineering. Because the board is backwards, the power cable is on the wrong side, causing it to cross over the SATA cable. REAL POOR engineering.
  - Unlike the older version, this one is a bit on the flimsy side, and difficult to seat the end caps.
  - Unlike the older one (which remains cool to the touch after hours of running hard), this one gets pretty warm (not hot, so acceptable... but remember, heat slows down electronics).
  - Basically, apparently Rosewill's CEO doesn't make enough money, so they outdid themselves using CHEAP parts and engineering to design and build this unit.
  - I'll update this after doing some speed tests. I'm especially interested in a comparative analysis between the old unit and this new one. It should be twice as fast (minus 1gbs, because I'll be using a pcie card for eSATA... it's a bus speed deal). We will see... 


It doesn't seem it comes from an electronics expert, although he mentions heat as a factor (no, it doesn't "slow down electronics" in general, it can damage it!)
And that rant about cables going "the wrong direction" seems a bit of nonsense, from a technical POV. 

Disclaimer: I'm not dismissing nor supporting the view of that guy with respect to that product. I'm only analyzing his technical expertise in electronic engineering.