Short term contracts are a surefire way to get bad results. A dev that knows they don't have to maintain the systems doesn't have a reason to care about long term sustainability. So you have to have senior devs babysit them instead of writing code. Some seniors are fine with that, others will start interviewing elsewhere when they can't code.

There's also the old joke of "it'll take a month to get done. ok what if we double your manpower? Three months to get done".

Not to mention the multiple months of on ramp that happens as a dev learns what the codebase actually does.

After a hire I wouldn't expect them to be meaningfully architecting anything in a codebase for 2 months. Only doing minimal patch fixes, small 1 and 2 point tickets. A contractor would never be trusted with arch and would never graduate off those short tickets.