Questionable? I consider any tech that gives a free Great Person a priority. If I don't need the Great Person then I'm still denying it to my opponents by researching the tech first.

->

-> larger army, unit upgrades
I consider the ability to support a large, fully upgraded army to be important.

also -> larger army, and the conversion rate from hammers into units is a lot better than from money into units. And why not just support your army and upgrades with the 10,000+ gold that you are giving up in order to put the research points into Machinery, and get +10

from a gambling house while you do it? How many turns would it take to recover the cost of Machinery from the extra commerce from windmills? Assuming you build 25 windmills, which seems reasonably large to me, and each one is giving you an extra 2

over an alternative improvement, then 10000 / (2*25) = 200 turns for you to recover the cost of researching the tech. This does not include the time spent actually getting the windmills built, or the cost/upkeep paid on the workers doing it, etc. The free Great Engineer would help you recover only a small portion. Crossbowmen are decent, but if we're talking about huge late game armies, 4 extra units is not much of an impact.
So yes, I would say that Machinery is a questionable tech.
Arete is a nice civic, but most civs are probably not going to be using it, so considering it's +1

/mine as part of an analysis of the worth of windmills isn't very objective.
I haven't been doing that. If you
do consider the Arete bonus, then mines are equivalent to windmills (assuming 1

= 2

, which Gekko said earlier), and then the cost of researching Machinery has absolutely no payoff and is a waste of time and commerce. No, I've been officially looking at the numbers as though the Arete bonus is not there, but I have also been recognizing that its strategy is available far, far earlier than Machinery's. By the same token, not every player uses Air Mana, either, and making windmills a little more dependent on the player's strategy would make them more interesting to the game. Nobody seems to be arguing that Earth Mana makes mines too powerful.