long serves vs short serves

1) There's much less options and angles available for the opponent when you serve fast and long as compared to short. A long serve can only be returned long (if they attempt to return it short it's gonna sit up high unless they have some Waldner touch), and either they push it (which results in a slow attackable ball), or they loop it (in which case you have to defend well). Whereas for a short serve, the receive can be anywhere from a short push to fast long pushes to flicks to chiquitas to sideswipes, and theres a lot of ultra wide angles that can be used against you. 

2) Long fast heavy sidetopspin serves just can't be pushed unlike short sidetopspin. This is very useful in forcing a topspin to topspin rally if you know you're favoured in that department.

3) it's the perfect antidote to flick/chiquita happy ppl, because the long serves test their footwork and response times so they can't attack strongly from the get go. If they step in too early they're gonna be super out of position against the long serves.

4) Forcing a loop from the opponent can be very beneficial if you know their patterns well, as you can anticipate and then angle the block from their loop well. It also saves a lot of energy as you don't need to loop yourself. 

5) there's simply less response time available for the opponent, if they're slow you can even ace them sometimes. It's almost impossible to ace someone with a short serve (misreading spin doesn't count)

6) the effect of spin on the rubber is more pronounced at higher speeds, there's more energy on the ball which induces more direct errors from the opponents if they read it wrong. With short serves usually a misread causes a pop up which you still have to kill (and you still have the opportunity to miss or they can block your kill shot)

Of course if the opponent takes a step back due to the long serves, short serves become killer once more especially when they have to reach for the shot due to being out of position. 

But in general, I think unless the opponent is reading the serves perfectly and punishing the long serves effectively, there's not a lot of reasons that short serves are better than fast long serves

Basically, all services which bounce at least twice on the table can be called short services.

To me +/-70% of the pro services are "short" services with the second bounce landing +/-10-5 cm before the end of the table or just behind.

Very short services are often really easy to return for high level players and often, the server loses his initiative and gets a chiquita difficult to deal with. 

Nearly all male pro players are able to make chiquitas nowadays, the only very short services which is often quite effective is on the FH side of the receiver, near the side line, with a sharp underspin or YG services with lateral topspin.

For very long services, often, pro players use them once in a game because it is really risky...or you have a service like He Zhiwen...

Yes. (I can't get the guy coaching me to realize this-they serve long to my BH where I have LP. Even flipping my blade is not much help).

Agreed, it's really not that easy to win the point outright with the 1st loop these days, especially when the long serve is of very high quality (fast, long, low, good placement, plenty of spin variation) which makes it very difficult to add quality to the loop. 

The advantages of long serves are even more pronounced against pip players because they don't have the advantage of generating heavy topspin in their attacks thus making it much more unreliable and mistake prone. The best response they usually have is to bump or chop/push it back low and long - and then this gives you the chance to comfortably perform an attacking loop which just starts the rally. 

I remember watching Sun Yingsha playing against Ito and it was just long serves all day long with the occasional short no-spin serve just to force the rally.