I tend to agree with you - churning the United card isn't possible because its issuer (Chase) is competent in preventing multiple bonuses both due to 5/24 and frequency restrictions. AMEX has 'once in a lifetime' rules for all its Delta cards.

What pisses me off more than anything about this - more than the potential loss of the actual miles - is that I am fed up with corporations blaming customers for taking advantage of their own failures, in this case, Citi's inability like every other bank to place effective frequency restrictions on earning signup bonuses. Not only did they not have a frequency rule for these offers (NONE of the mailers had 24 or 48 month language), but their credit analysts knowingly approved people seeing credit reports with 5, 10, 15+ of the same card, and approved people regardless of who applied vs who was targeted. When there are loopholes or offers that are 'too good', some people are going to take advantage of it, period. It doesn't help that they have cultivated an antagonistic relationship with their customers to begin with (airlines and banks are notoriously unpopular outside of forums like this), so people aren't going to feel remorse about taking advantage of loopholes against these companies.