We are heading to WDW this August and I am contemplating buying Disney Gift cards at Target using my Target Red Card to get a 5% discount. Our trip costs roughly $5000 so this would be a $250 savings. Any downsides aside from if we cancel (extremely unlikely) I’m stuck with a lot of Disney gift cards?
Lots of people do it. There's an entire thread on the Budget Board dedicated to gift card discounts.
Downsides would be the $$$$ sunk in Disney GCs which are non-refundable.
Then there's the problem with combining GCs - you can only merge up to $1000 on a single GC using the disneygiftcard.com website.
Hacking of GCs can occur and straightening it out can be a hassle or may not get resolved at all.
GCs can get misplaced or lost.
The Disney website may limit the number of GCs that you can use in a single payment. Calling to pay with multiple GCs can be time-consuming.
If you cancel a trip which was paid for with GCs, the funds get returned to the GC that it came from. It's important to keep that GC info until you have completed your trip. For that reason, you also do not want to keep transferring balances to the same GC over and over in order to make multiple $1K payments. The maximum value that a GC can have is $1K and Disney would not be able to refund $5K to one card. That would require them to send you 4 $1K GCs and that could take a lot of time.
ETA: I buy Disney GCs throughout the year in order to pay my DVC dues every January, so I'm not against purchasing them. But you asked for the downsides so there you have them. BTW, If you're really good at it, you can do much better than a 5% discount.