The Tongue is mightier than the Sword.

Loshen Hora.

This is one topic that is very relevant to shidduchim.

It is a very complicated topic that is difficult to cover because there are a few diverging opinions about what one can say and what one cannot when talking about shidduchim.
Some Rabbonim will be more stringent and some will be makel.
The following are just basic guidelines, and are here to give you an idea of what are the issues, but every question should be referred to your Rov. There are a lot more issues than are covered here and every case has to be decided according to the circumstances.
Halacha differentiates between 3 conditions:

The first condition is when a shidduch is first suggested.

At such a time the person suggesting the shidduch should be careful about “Lifnei Eiver Lo Sitein Michshol” “Do not place a stumbling block before a blind person” Which applied in this case means one should not suggest a shidduch that would not meet the needs of both parties involved. It has become very common these days to hear of shadchonim or friends just naming names to a parent as possible shiduchim without really taking the time to discover if there is real reason to pursue such a union or not. No individualized thought goes into it. The parent is told: here look into Plony, Almony, Tom, Dick and Harry. Are any of these compatible? Are these families appropriate? The person suggesting the names does not know, they just throw out a bunch of names. Suggesting a shidduch to a possible family without checking if there is compatibility, or if the shidduch satisfies the needs of both parties, is not allowed. The parties will waste time researching a shidduch which should not have been initiated to begin with, and gives false hope to the young people involved who will be disappointed at the falling apart of a possibility.

One should not refrain from suggesting shidduchim because of this halacha. One does not even have to investigate the parties thoroughly first before suggesting a shidduch. One does have to have some knowledge of the parties involved and be fairly sure that based on that knowledge this is a viable shidduch, and it has a good chance of succeeding. One should be reasonably certain that the parties do not have characteristics which are objectionable to the other party or that one party is lacking a non negotiable quality sought after by the other party.

This restriction has two main points:
The first is wasting the time of the parents and young people involved.
The second restriction concerns information: sometimes when we suggest a shidduch there is some information that may be held back for the sake of making the shidduch. One cannot decide by oneself when one may conceal information and when one has to disclose it. There are different kinds of information given by Chazal, “major deficiencies” or minor problems. A Rov must be consulted to make sure the case warrants keeping the information hidden altogether or at least for the beginning so that the shidduch has a chance of succeeding.
Even this deception though has to be revealed before the parties see each other enough to have developed feelings for each other.