Boxer engines are inherently balanced, which means they don't require power-sapping balance shafts to prevent them from shaking apart, but they aren't perfectly smooth in operation.
The Wikipedia article on engine balance briefly mentions this:
The boxer engine is a type of flat engine in which each of a pair of opposing cylinders is on a separate crank throw, offset at 180° to its partner: if the pistons lie on the same axis then the design is inherently balanced for the momentum of the pistons, otherwise the design has reciprocating torque (also known as a 'rocking couple') due to the cylinder axes being offset.
There are also factors external to the engine (like motor mount and exhaust design plus NVH-reducing features) that affect the perceived smoothness of the car.
Subaru engines are nicely mechanically balanced, but the cars themselves tend to lie a bit towards the hollow rattle-trap end of the spectrum which does not really support their marketing.
Porsche engines are similar. Nicely balanced by design, but they sound like a bag of loose parts when driven in anger.
Feeling a little bit of boxer shimmy at low RPMs is a normal part of the Subaru experience -- especially with EJ engines sporting unequal-length exhaust headers, where it's very noticeable.
Lots of shimmy could indicate a misfire, malfunctioning idle control, or a broken motor mount. That would need to be looked at.