Providing a Shield Guardian

Hey folks, my players are about to encounter a boss fight where a Shield Guardian and its amulet are involved (part of the Phandelver and Below for those who know).

I kind of expect my players to go for it once they realize what it does. Has anyone run into issues with one of these "joining" the party? They guys will have hit level 7 after that fight, so it being CR 7 should be roughly ok, although still pretty formidable in general for a bit. 

Do you typically leave it fully under the control of the amulet holder? How is it controlled? The encounter stat block has the boss use his bonus action, for which the guardian is an eligible target to be forced to operate, though it can also control other creatures. 

I guess eventually it will lose in efficiency and be left behind as it won't be leveling up alongside them.

Anyway, i was curious if anyone had suggestions on this thing.
Thanks.