Ignore the obvious problem of Sybil attacks, especially those exploiting BTC's current crummy P2P code and superlinear vulnerability to maliciously construed (ie very slow to verify) blocks.
I just wanted to come back on the superlinear problem that iCEBREAKER mentioned here, because it deserves response since it's just as much a problem for other BIPs and SW.
So this is not a new problem introduced by BU, and there have been some proposals made in the context of BU to mitigate this (by moving verification into separate threads etc.)
Most of the discussion around this in BU has happened in the thread below as far as I have seen:
https://bitco.in/forum/threads/i-really-want-to-like-bitcoin-unlimited.684/page-2#post-8087