Bitcoin Unlimited is highly experimental and many of the basic operating principles are at best speculative. They should try it with an alt-coin fist at least, or a sidechain.
BU nodes hum along in the main network just fine, no one
is hurting themselves or anyone else.
But for studying longer-term perspectives, i agree it would be a
good idea to make an alt without fixed blocksize limits.
But we have a problem here; how to simulate adequately Bitcoin's situation
where non-mining validators blocksize preferences are limited by their
computing resources while miners preferences are subject to considerations of
a different sort.
Again this emphasizes that the problems that BU tries to address
are not just technical but more like socio-economical.