To an extent fielding your Ultramarines as Iron Hands goes against WYSIWYG. The book clearly labels things like special characters for what chapter they go to in the army list down to the chapter icon. Giving Pedro Kantor a Crimson Fist icon shows that while he has same chapter traits as Lysander, they can't be fielded together. This also in my mind pushes out using them in successor chapters, custom or otherwise. I don't want to have to explain that my Imperial Fist army uses Imperial Fist chapter tactics at every tourney. That should just be obvious by the color and chapter symbols.
Now beyond just the counts as in the Space Marine armies, people who do things like run Chaos Space Marines as Blood Angels to use Khorne Berserkers as Death company. All these have their own clearly defined rules and asking your opponent to just forget that and try and remember what each of your counts as is instead of isn't can get tedious. Chaos players seem the most prone to doing this since they resent their codex for whatever reason (no idea), and are mad they don't have an equivalent to chapter tactics. This I can sympathize with. You might not want to wait for GW to release supplements for Iron Warriors or Night Lords. But running them as Space Marines? That's not Chaos, that is heresy against Chaos. (Iron Warriors as Imperial Fist is extra heresy)
Yes I do cherish fluff over rules, and I know someone will disagree with me on this whole subject. But it makes for good debating material.