SixTen could you back that up? Otherwise do no make comments like that!
Many of his dawah, regarding the Qur'an, is utmost contradictive of what authentic tafseers state - as well as contradict many of the greatest scholars. Yes, he seems so good, when you do not have a lot of knowledge about Islam, but if you took your time to study the Qur'an, regarding what he preaches, you will find him not being just wrong, but intellectually bankrupt on many of the things he says and how he justifies oh so many things.
To put it simply, he has just made his own version of Islam, if you know what I am saying. In that, he makes verses mean things which they do not. Sure, people try excuse him with, verses could have several meanings. But believe it or not, the Qur'anic verses were given with meanings - and its one thing to have 1 verse mean a lot - but to mean contradictive meanings is an insult to the Qur'an.
Most people start getting into Naikh, in the teen years, as I did. I thought he was the grand master of dawah. Until you saw, everything he says has been refuted. Not just by non-muslims, but also
MUSLIMS.
Their are things, he does say right and answer correctly. But I think, he has, imo, robbed muslims of real Islam, you can't so easily forgive him or, call him a speaker. Its more than just a "mistake". Sadly, from such roots, we have the weak dawah being spread all over the world - for many young muslims. This in turn makes them give dawah which is actually isn't true. What good is it if you convert muslims over somthing which isn't really factual? Ontop, alot of the arguements, would just make intellectuals laugh - as they really are that bad.
As for Ahmed Deedat, he doesn't get the same treatment as Naikh as, as far as I know, he hasn't "butchered" the Qur'an. He does comparative religion a lot. Again, although his arguements seemed great when I was young, having learnt a bit more about several faiths, I do feel that he does to Christianity, to say, what Robert Spencer does to Islam. Sure, Robert Spencer seems to make perfect sense to non-muslims about Islam, but that is because they are non-muslim. I have found people converting to Islam over his work, but then I know people who left Islam over works criticising Islam. Does this mean either of the dawah was factual? No not at all, just that it convinced the person. Personally, both sides actually use very weak arguements.
Want to learn about Islam? Go for the real stuff, not this modern re-translation of Islam.