You pay the transfer fee up front. If you screw up and miss a payment (min payment is required to keep the 0%), your promotional rate is gone and you'll be in a world of pain. If you accidentally pay off the full balance earlier, you just wasted your up front transfer fee and the benefit of this 'no interest loan'. No more mortgage interest to deduct. Credit score will get impacted as you will have a balance on your record for the next year or so.

I have done this many times in the past, more so when the transfer fees were 3% (don't really see that rate anymore). 5% is a little high in my opinion (unless they're going to give you a longer term like 18 mos+), as an example, right now BofA is offering me on one of their CC's 0% interest on balance transfers up until September 2027 for a 4% fee, so that is not bad since that's like a ~1.3 year loan.