I think MoarVM/NQP are strikingly similar in their approach based on what I've learned so far about Graal/Truffle (which isn't much; perhaps a few hours of exploration; eg reading/watching the One VM to Rule Them All, One VM to Bind Them paper/video). 
Given the large number of Oracle employees working on Graal/Truffle (40 in 2016 according to the video) it doesn't surprise me that it's currently a lot faster.
Did you use Graal/Truffle friendly benchmarks (just as was done in the video)? Have you tried MoarVM friendly benchmarks to see where MoarVM shines? In particular, have you accounted for the new JIT that recently landed after 3 years of non-master branch development? 
Would you please elaborate a little on how MoarVM is insufficiently flexible?
TIA for any reply.
Does MoarVM use a meta-compilation technique? Pypy for example uses meta-tracing and Truffle uses Partial Evaluation to get from an interpreter specification to dynamically compiled code without duplicating the language logic. If yes, could you please point me to further information?
Meta-compilation is the primary innovation in Truffle. It allows to compose languages together and to compile them as one unit.
If you want to know more about the Truffle approach to meta-compilation see this PLDI paper from 2017.
Its NQP that does metacompilation, not MoarVM (afaik).
I linked to NQP's (spartan) readme above.
Some other points/links that might be helpful:
* nqp is written in nqp.
* nqp has a first class grammar construct. See wikipedia's Perl 6 Rules page. (nqp's grammar is defined using nqp's grammar construct. So is Perl 6's.)
* nqp and MoarVM implement a protocol for abstracting behavior (eg methods) and representation (memory layout and use) called 6model.
To try set your expectations appropriately, let me emphasize this note from the NQP readme: "NOTE: there's no end-user support for NQP and the behaviour can change without notice. It's a tool for writing Perl 6 compilers, not a low-level module for Perl 6 programmers."
NQP is a tool for writing compilers, not just Perl 6 compilers, and is in fact much more than that, but the Perl 6 project is currently very focused on the Perl 6 language and definitely doesn't want ordinary Perl 6 programmers thinking NQP is a tool for them.
Doc is sparse, again reflecting the focus on the Perl 6 language and the Rakudo compiler that builds atop NQP, analogous to your team focusing on just one of your guest languages.
The best doc for getting into NQP is not even in the NQP repo. It's the support materials for a 2-day workshop held in 2013. The abstract is "This intensive 2-day workshop takes a deep dive into many areas of the Rakudo Perl 6 and NQP internals, mostly focusing on the backend-agnostic parts but with some coverage of the JVM and future MoarVM backends also. During the course, participants will build their own small compiler, complete with a simple class-based object system, to help them understand how the toolchain works.".
Beyond that, there is a doc directory in the nqp repo.
Please write an update here on what you find out.
(nqp rules are Perl 6 Rules grammars but the general purpose DSL (!) for writing procedural code embedded in an nqp grammar is nqp's general purpose DSL rather than Perl 6's general purpose DSL.)
(MoarVM directly implements 6model -- MoarVM is named for "Metamodel On A Runtime". 6model is how one nqp based language is able to do magic like subclassing a class written in another language (whether nqp based or via an existing compiler) despite them having distinct OO semantics and implementation strategies.)