You can have your own "disposable mini-squad" with the assassin implant... as a engy.

Gentleman... today I bring you another "Things that the game doesn't tell you but it's there" secret... which as title implies... your own disposable mini squad.
In short: Assassin implant gives you the ability to make everything you place around the world a extension of you. Spitty becomes you. The AV mines becomes you. The AI mines becomes you. So we are talking about 9 (yep, nine) possible damage sources that will act as "literal pocket BR 10's" that will damage -and- spot anything that it touches. Which means, if you place five AV mines on the ground close to your ANT-made base while your friend babysits it... you can safety spawn elsewhere and let the mines warn your friend if there is a threat nearby. Even if the mine doesn't kill it -- your friend definitely will. Or you can place a spitty and warn everyone -INSTANTLY- when a cloaker is about to stab your whole squad.
There's also another neat thing... it works for EMP nades, as well. Which means, if your empire is facing a enemy zerg, everyone is pinned down and you need to know how many zerglings are on the other room... just throw a emp in the middle of said room and -everyone- will be spotted. Considering the emp nade has a huge aoe... yep, you don't need recon darts anymore.
It can also remove spotting from you -- if one of your mines, spitty, anything else gets a kill. Mwuehehehe

I wonder if it works with the orbital Strike....

It does. :^)

Does it spot guys who made it inside a building?

Every damage type triggers the implant effect (even if you damage something by 1) -- so yes, it will spot everything inside the building.

Everyone already has access to crossbow recon bolts. We also have modules that deal as a warning system for player bases. We also have implants that should invalidate Assassin to a degree such as Avoidance which is useful on its own. Additionally, your minisquad is a fairly blind creature since parts of it would be unable to react to the enemy's evasiveness, being rooted in place, and other parts (some overlapping with the first) would be expensive to setup and maintain.

Using an implant slot to facilitate this seems like a waste of slot - I say that as someone who does not use implants - unless it had some alternate benefit. It might show some luster at times but it doesn't sound like something that would become a mainstay.

I'm sorry, but 9 damage sources that will spot and identify threats for you is no way a "waste of a slot". I mean, it's the only implant in the game that allows you to place a spitfire inside a cap room and use it to identify whoever gets in the room while you hide elsewhere, safe from harm and with an advantage over your target (since you -WILL- know where your target is and its facing direction -- something that the crossbow recon dart does not allow you to.). Which is better than gaining 200 shields (and nothing else) after a headshot kill. Or a healing effect that will kick in after 8 seconds and takes forever to fully heal. I mean, we are talking about a class that has zero abilities with combat advantages.

It's a force multiplier, just like Assimilate with a Lasher. You sacrifice some battlefield effectiveness so that the rest of your squad can reap the benefits of all those spots.

More like a gimmicky cloak of sorts -- considering what I said above about using the spitty as a decoy while you hide on a corner so you can take a (proper) initiative.

I said that it "seems like." In a sense, it might be, compared to other methods. With an Engineer, you have access to infinte ammunition and that means your recon bolts are fully replenishable. At the same time, they're no less obvious when in use than Beepy. The utilities required to make the implant effective are finite save for the Spitfire, and the Spitfire falls short in terms of identification of large groups of enemies.

A class without combat viability is a class that is not being played to its strengths. Granted, your suggestion is a good demonstration of making the best of the Engineer's gadget arsenal, but I believe it's an underwhelming result for the required setup. At best, it sounds like a learning tool to instruct inexperienced Engineers how to treat their explosive/response arsenal as an extension of their senses, an important skill that doesn't require the implant (though does require paying attention to sounds and spatial awareness and the implant sounds like a good shortcut for those).

Hunter + recon darts means that you won't be using underboss, a commie, or even a shotgun (if you have the "secondary shotguns" ASP unlocked). Even if the spitty is "too obvious" and won't be able to identify multiple threats -- your AI mines will. And I'd rather use a implant slot to (proper) identify targets instead of using a secondary that does a poor job compared to it.

I don't think "ingame experience" will allow you to somewhat cloak inside a cap room while your target ignores you, allowing you to take the initiative. Which is what you can do (theoretically) with the spitty + assassin implant.