Tenser is a brilliant spell. Someone should write a handbook on its uses.
I don't think the relative weakness of the individual servant is a particular limitation. Since they don't take up space, there is no appreciable limit to how many at most you can have exert force on a single thing either, although they do need to be given the order individually. Another point is that their ability to use tools and "mend" should allow for the use of crowbars and stuff to generate mechanical leverage that would help. Very well within the realms of what an untrained human can do with a DC 10 check of some kind.
What I think limits them for this sorta labour, particular transporting things like you mentioned, is their measly 15 ft speed and lack of brains. Going by the FAQ, they lack sense or judgement, basically being limited to differentiating things by size and weight (i.e. what you could perceive by touch), so giving orders that amount to effective deconstruction is tricky. So you can probably give an order that would amount to them prying all the nails and screws out of the room on their own without active directing, but without direct guidance they couldn't pull apart interlocking beams. An order to just indiscriminately pull things apart into whatever direction they can move might be more efficient. Or use saws, if there were any...
Other indiscriminate building destruction options would be to have them move the earth out from under the building until it collapses, or have them in groups of 10 lift 200 lb object and drop those from like 40 ft up over and over until everything is broken. Doesn't even need to be for a purpose. Stroll through town and have them wreck things all around you for giggles.
Fabricate is better in a lot of ways, but is also a 5th level slot and has a volume per casting limitation (especially for mineral stuff). The math will depend on what you're working with, but for most purposes the number of Unseen Crafters you'd need to out-pace it should still be in the double digits.
Also, I don't think they are necessarily limited to making a single item, they just have to keep crafting the same thing for the duration, e.g. churning out arrows, as that is still "one task that can be accomplished by the use of the Craft skill". Regarding ranks, considering they add your casting stat and can take 10, hitting the 15 for martial weapons or "high quality" items should be doable without rank investment. If profit is the goal, the obvious pick would be poison, where the number of doses you make per week gets calculated in gold rather than silver.
Rather than making cash though, I think the more interesting use is making cheap stuff and leveraging the sheer volume you have for in-game purposes. Like, turn a helpless village into a militia decked out in simple but effective leather/hide armor, bucklers and Longspears. Of course fortifying the whole place with walls and watchtowers can be done at the same time. All you need is a Goblin Army Ex Machina and you'll be a regular General Enri of Carne Village.