In every way possible!
The biggest stress on a Bi-vocational pastor is time. There is so much we want to do, we should do, and we are called to do as a pastor, but time just doesn't permit everything to fit. This can be a stress on you and your whole family (very important to specifically carve out some time for family, otherwise it's them that will get overlooked, and that's not good).
A GOOD associate pastor who is dedicated to the ministry and working along side you can double the time spent on ministry without killing you or your family, but it has to be someone who will work WITH you, and you can trust fill the role as pastor when you are not available.
Now if you are speaking of more an administrative assistant, That's harder to say, if it's only in the paperwork and "business" end of the church that could be a help if that is tying you down, but you have to be careful that you aren't spending just as much time getting the information to the person, and proofing it as you would just doing it yourself. This probably depends mainly on how much administrative work you are doing.
"Why would God allow Hitler his chapter in history? Why would God send a whale to swallow Jonah or turn a woman into a pillar of salt? Surely there were other ways. But I'll leave the particular methods of God to God."
-Spoken by David Abraham, Showdown by Ted Dekker