if using a mixing site, isnt it possible that someone elses "tainted coins" could be sent to you, personally i think exchange might be better.
There is nothing wrong with the tainted coins if you can spend it. The coin from exchanges are also tainted.
Yes true. There are huge amount of coins which are tainted due to robberies or hacks which happened before.
However, companies may reject those kind of coins. For example, Bitpay has allegedly[1] warned the merchant about the blacklisted coins and it makes no sense whatsoever. Almost all the coins are tainted in someway and Bitpay would be marking majority of the legit transactions as a fraud.
[1]
https://www.reddit.com/r/Bitcoin/comments/3mea6b/bitpay_is_blacklisting_certain_bitcoins_rejecting/