Part of the power of Proof-of-Work is the asymmetry between high cost of generating valid data, and low cost of verifying it; anything that reduces this asymmetry is arguably decreasing the security of the system.

If there is greater uncertainty during the early days after a hardfork about whether some transaction could get confirmed, then your fully-verifying node will suffer increased churn of the orphanage, and your resource consumption will suffer. You might not take an invalid payment, although your operating costs still go up, to the point where it could be rational to declare that you have enough faith in your side of the fork to pause your node for a few days until the chains have forked so far apart that the uncertainty is reduced and your verification can proceed efficiently.