Lack of enforced hygiene, for starters. Hygienic R5RS-style macros are nice and all that, but only for petty language extensions, and they're a noticeable obsta

Lack of enforced hygiene, for starters. Hygienic R5RS-style macros are nice and all that, but only for petty language extensions, and they're a noticeable obstacle if you want to implement more complex, deeply staged DSLs. It's possible to find workarounds for most of that things, but yet, not having hygiene in the first place is better.