Ya, certainly. I mean, I think the most regressive part of the system that almost nobody talks about is the fact that unrealized capital gains compound untaxed. People focus a lot on the differential treatment of long term capital gains vs income, but I think that mostly misses the point. The real wealth building effect from capital gains comes from allowing them to compound completely untaxed for decades, if the gains are never realized. Unfortunately there's not any obvious workaround for that that doesn't have serious negative consequences.
Similarly, including or excluding donations, inheritance, and home sales from income can really change these numbers. I have seen analysis that says are system is progressive, regressive, and flat based on different definitions.
Honestly, that’s what I find most interesting about the system. The collision of a huge number of different interests result in a system that can be perceived mathematically different based on viewpoint. It’s like watching a magic trick in action.
PS: One of the most extreme divergent analysis focused on the sales and sin taxes paid by a group of homeless drunks. It varies by area but with 40+% percent alcohol taxes they where paying a rather shocking effective tax rate when you exclude benefits.