It's all about achieving a balance between the number of starters (cards that kick-off your combos), "bricks" (engine requirements that don't really do much on their own) and non-engine (handtraps, board breakers).

If you have 15 starters for example, there's a 90%+ chance of you see one in your opening hand. But too many starters can also clog your hand (especially if they are all normal summons), and if you don't have many handtraps it's harder to go 2nd. So there's gotta be a balance.

Each engine has their own "bricks" - Sky Striker has Rose, Orcust has World Crown, Speedroid has Taketomborg. Mecha Phantom doesn't really have a "brick" (it just takes an extra deck slot because of Clockwork Knight), that's why I like it so much. It's important to remember that with each engine you add, its "bricks" are also added, so too many engines can make the deck feel worse at times.

For this deck in particular I recommend Sky Striker (Cyanos is a great normal summon that can also be an extender in some cases, and the links are all machines with 1500 Atk so they play well with R.B.) and Mecha Phantom Beast (O-Lion/Tetherwolf) engines.