Well, runtime/warmup costs seems like one obvious downside to me - weval would add some non-trivial compilation overhead to your interpreter (unrolling of interpreter loop, dead code elimination, optimizing across opcodes boundaries - probably a major source of speedup). Great if you have the time to precompile your script - only have to pay those costs once. It also helps if your host language's runtime ships with an optimizing compiler/JIT you can piggyback on (WASM runtime in weval's paper, JVM in Graal's case) - these things take space. But sometimes you might just have a huge pile of code that's not hot enough to be worth optimizing and you would be better off with a basic interpreter (that can benefit from computed gotos or tail-call dispatch with zero runtime overhead). Octane's CodeLoad or TypeScript benchmarks are such examples - GraalJS does pretty poorly there.