The point is, these tests are not enough

I would encourage you to right GitHub issues to cover specific test cases that are not covered by the existing tests.

What exactly makes it challenging ?

Hardfork deployment requires a lot of coordination with exchanges. As can be seen with HF20, there are several major exchanges that still have not gotten back online after needing to upgrade.

Even non-consensus changes (such as 20.5) often require exchanges to upgrade and replay.

The TLDR is that blockchain deployments, which require coordination among lots of different decentralized parties in order to successfully do, are not the best suited for doing frequent small deployments.