@desolutionist Pack Rat was good before ballista was printed, now its just pretty bad in my opinion.
In general splashing 1 additional color for eather Mentor or Wheel and Empty is pretty easy and there is just a bit downside to it. However, my current build is pure UB and features tinker bot in the maindeck instead of Mentor or Empty. I dont really miss wheel a lot, i have to say, but wheel would definetly be the main reason to but a badlands in there again, not really Empty. However i dont object to empty as much as i did a few month ago, i can see it as a small number, what alienates (get it?) me is people playing 3 or even 4 copies.
Against Shops i rather board in a huge amount of bounce spells and aim to win with 1 of them resolving at the end of their turn, or try to win early with a tinker. ( i play 1x Hurkyl's maindeck, and 1 chain of vapor, and i bring in 3x hurkyl's, 1x Rebuild, 1x Echoing Truth and 1x Island. cutting mostly discard spells and defense grids)
Against most blue control decks i board in more defense grids and sometimes more discard (depending on the state of my sideboard at the time).
So generally i much rather board in cards that force through my 2 main win cons (tendils and bot) instead of boarding in additional win cons.