I'm not sure why there is so much concern about balancing for separate stats on this thread.
Let's look at what the engineer has to give up just to use a mech:
traitline (mechanist traits are mech only)
toolbelt skills
In PvE, the mechanist and mech will use the same stats or two complimentary stats for the same role to be optimal. A berserker mech and minstrel engineer is not going to somehow break PvE or any other split role build. The engineer gives up too much to use the mech to fully function as a separate role. The mech AI and current abilities make it incapable of performing a player role on it's own. Also, you would lose benefits from traitlines for either the mech or player based on what role the traits are supporting. Ie focus on a minstrel healer engineer with healing traits and the pet loses dps traits from other lines. The mech is also going to spend a lot of time dead with the current ai and only the elite signet to save it. There is no way the pet will bring enough dps on it's own to secure a slot in any group content.
In PvP, we already have the ranger which is similar, especially the druid which has basically capitalized on this since their release. The difference being a ranger is still essentially a full class without a pet. They have 2 weapons and 3 traitlines to benefit from even if they lose their pet. The mechanist on the other hand, is a kitten class with 2 traitlines, 1 weapon and no toolbelt skills. If they fully focus on their pet with a split role they have no trait benefit for their role, 1 weapon and no toolbelt skills. The mechanist pet may be stronger than ranger pets, but it needs to be otherwise the mechanist would have no ability to compete with other classes. The core engineer is not designed to lose a traitline/toolbelt skills and still be competitive. On top of all this, PvP is still balanced independently of PvE, so they always have the option to adjust specifically for each game mode.