Maven allows interactive development too when using the IDE integration. An IDE is in a better position to have that kind of information and integrate it well (e.g. an IDE can run tests on save without having to watch the filesystem for changes).
SBT is not adequately documented, relies too much on symbol-heavy syntax, and allows arbitrary code in build definitions.
XML uses three symbols: <, >, and /. And it's a well-known, documented standard. Looking at the last project I depended on, its build.sbt uses :=, %, %%, /, <+=, <<= (in addition to standard Scala symbols like +=, ++=, ++, =>, <=, >=, which an inexperienced developer wouldn't necessarily realise weren't part of SBT's own syntax). That's not an exhaustive list (I've used :+= and :++= as examples in another thread) and I have no idea where to find one. Heck, I've been a professional Scala developer for 5 years and I have literally no idea what <+= or <<= mean.
And there is an oft-documented overuse of custom operators among Scala programmers, however I don't have sufficient expertise to gauge the resulting problems in practice.
SBT is not adequately documented, relies too much on symbol-heavy syntax, and allows arbitrary code in build definitions.