It is the miners that create new blocks, that is what counts, nodes simply choose whether to follow the miners or not. Nodes can be spoofed, proof of work can not, this is why it works.
So in other words miners make the ultimate decision? If this is the case, then Bitcoin Unlimited is even worse than I initially thought. Otherwise, elaborate what you mean exactly (since everyone can vote via software and miners create blocks).
I do not think that the miners having a lot of power in this decision making process is a bad thing, after all they are the only group that have a direct and lasting incentive mechanism acting upon them to do what is best for Bitcoin. Furthermore this would not just be up to the miners, they would still need to move with the economic majority, this is where the negotiation would take place. I also do not see a better alternative then that since otherwise we would still be dependent on alternative implementations and their development teams in order to give us a range of choice. In that case however the power would still lie with the economic majority and the miners so it would not actually in effect be any different in that regard, it just turns what was a representative democracy into a more direct form of democracy in regards to the blocksize. BU is not even about increasing the blocksize, it is about giving everyone the freedom of choice. After all the miners and the economic majority could decrease the blocksize under BU if they deem this to be necessary, BU just takes this power away from the development teams and returns it to where it belongs.
BU can even be configured to mimic the behavior of BIP100, BIP101, BIP102, BIP103 or BIP202. Or it can be setup to behave exactly the same as a Core node as well, the choice is yours, in different words it is up to the free market. Considering all of the possible options of who decides I would argue that this is the best solution.