I feel like that's largely mitigated by profiles. RVA23 is really looking like it'll be the modern base target used for high performance application processors and it makes mandatory pretty much everything you'd want for those use cases, and other comments by people familiar with designing RISC-V CPUs mention that the variable length encoding can be dealt with in a very simple manner that doesn't even add another pipeline stage so it doesn't seem like it's all that big of a deal while also bringing in benefits in code size reduction. Not everyone is adopting it, but several major players have set the stage by mandating it.