Lingual braces are completely safe which is why it has been introduced for treating the general population.
But there is a catch.
It should done by an expert. Not all orthodontists (braces specialists) can perform the treatment by lingual braces. Kindly visit an orthodontist
who specializes in lingual braces near you as they can give you the best advise.
As for it being completely invisible, that it serves the purpose since they are fixed to your teeth on the inside so that it is not visible (unless they take a peek inside)
There is another alternative for which you can visit a Dental Surgeon
who incorporates the latest advancements in their practice- It is called INVISALIGNERS. This can be used to treat mild imperfections in the alignment of teeth and this is COMPLETELY INVISIBLE. Although it takes some time getting used to. Also you can use it very conveniently.
Hope this answers your query.