Generally speaking is it true that lawyers don't want their clients to testify? Especially under oath? Why are they so loathe to do so?

 Do all clients lie? Isn't everyone innocent until proven guilty? Presumption of innocence? Why can't an innocent person testify? Now a pathological liar?  Better he/she should  be kept far away from testifying. But  a typical Joe who is honest and truthful. What's the problem?