Ethereum split into ETC and ETH demonstrated that cross-chain transaction replay becomes a major problem if not planned for. Transaction created on one chain could be replayed on the other chain, moving both types of coins, perhaps contrary to the user’s intention.
Unfortunately, this problems does not receive adequate attention when people are discussing non-consensus fork of Bitcoin chain. Imagine that as a result of fork, you got both BTC (core bitcoins) and BTU (unlimited bitcoins) on the same addresses. Now, you want to pay for something with BTC — but to your surprise, the same amount of BTU is also moving to the seller’s address, even though they were not supposed to. Another example, you don’t believe in BTU value and want to get rid of them in OTC transaction. You get the payment and transfer your BTU to the buyer. What the hell?! Your precious BTC from the same address are now gone as well! What’s the deal with that?!
Imagine the confusion among Bitcoin users that may follow and the damage to user confidence and willingness to transact on-chain just this single problem could do. Unlike Ethereum, where the replay problem was mostly dealt with via splitter contracts, Bitcoin does not have adequate scripting capabilities. The only practical way to mitigate the issue would be mixing the legacy coins with post-fork mined coins of different flavors (either BTC or BTU). Which is well beyond the abilities of casual Bitcoin user.