The unibody design makes the case lighter and stronger. Since it is machined, there is minimal error in the creation of the case. For more information, watch the video on http://www.apple.com/macbookpro/design.html. The first unibody MacBook Pro was the 15" model released in 2008 and had a removable battery. I believe that all other unibody models (and only unibody models) have integrated, non-removable batteries.
These sort of surgeries are always "twitchy" at best. Worse, the new butterfly key design makes these way harder. With the original chicklet style keys, a good technician would get 50 to 90% success rate. I would expect the inverse - high failure rates on the 2015 and newer key designs.
The tolerances in the keyboard area (mostly the scissors) are very, very thin - and at some point, extra hard superglue or some cup that's 0.2 mm to the left of where it should be will require you to get a proper replacement.
In the pre-unibody days, the keyboards were a single piece that you could easily exchange.
Now, the keyboard is screwed into the unibody frame, so it's a very deep surgery to get everything removed and then handle more than 50 exceptionally small screws to get at the back sides of the pieces where the rubber springs that yield the touch for each key are mounted.
Lastly - even if you do replace things - the keyboard action is not what it was (and most likely even close) - so if you're the type of person that would keep on typing even if the key cap was missing entirely - go for a repeat repair on the rubber nubbin with a razor blade and superglue. If not, it's time for a proper repair or using an external keyboard while you save money for a replacement.
If you were to get a precision knife to clean out the prior repair and choose a more flexible adhesive, you might have better luck.
Best Answer
The tag here is pretty authoritative to explain how unibody came about on the MacBook line where a united piece of aluminum was machined to house all the internal parts. This differs from the previous construction where discrete parts were assembled / welded / screwed and placed together.
This process is also used on non portable computers, but the difference in stiffness and reduction of size was most dramatic on the MacBook and MacBook Pro lines which then enabled the entire iPhone / iPad / Watch lines to evolve and refine what the unibody manufacturing changed on portables.
Apple doesn’t flat out define the term in the February 2019 Apple Style Guide, but the transition was when the original black keyboards arrived on the scene for the intel line MacBook Pro (initially) and later to the Air and MacBook lines. Currently all MacBook are considered unibody.