Portfolio ARM to good to be true?

[GL L.]

I have an opportunity to do and 5 year ARM with my bank with 30 year amortization. No Fees for anything as it is Portfolio loan and been banking with them for over 8 years. Bank Manager handling the loan.

It is the only type of portfolio loan the offer.   What are pit falls of  going this route over a 30 year conventional other than  interest rates possibly rising up to 2% every 5 years?  

This is My primary home.  Also considering this on investment rental income  property I own free and clear and 2nd home.  

Thanks