Are there significant downsides from enabling Metaflow cards (ex:

@card(timeout=600)

) for all steps for a flow? Do we incur additional storage costs etc?

cards are stored in s3 but the cost is very minimal

you will incur a small cost of generating the card at the end of your task. As @ancient-application-36103 mentioned though — likely minimal but it’s there. In your case, it would be at most 10 minutes delay. This delay would delay any other depending task as well (ie: the tasks that come after your task would have to wait for the card to be generated)