I am of the belief that Forest tiles should be able to be used to build both Tech based buildings - Lumber Mill (currently allowed) and City based infrastructure - All (currently not allowed).

Given that, if the Pioneers were to get permission to engage/build on Forest tiles (yaa!), perhaps a smaller Logging Camp version with Bonus (2) vs the full Lumber Mill with bonus (4), it seems only prudent to also allow Pioneers to capture the other  "City" bound,  via the 4 tile radius rule," resources.

Scenic Views, Gold Deposits, Wild Herds and Wild Flowers come to mind. They are all currently Tech based.  

Given that the switch to a Global Resource model certainly helps unencumber our Cities from being tied down to certain critical Resources (Food), then a total break should only add to that freedom of placement choice unless to difficult to implemented (unlikely).

The net result, imho, is that if a player really wants to make sure that a marauding enemy force doesn't easily take away a particular, and as yet properly guarded remote Resource, they have to make a conscience, and often tough decision, to place a City on that/those resources to assure its protection but, ultimately, may actually make the City itself, more difficult to Defend down the road.

Agreed, I expressed similar sentiments about pioneers building on forests in this post.

How about adding an additional cost to those structures then?

Woodcutting is pretty manpower intensive, so make a lumber mill built by a pioneer cost 50 people out of the next city. Or alternatively you need to feed those workers at the mill, so a lumber mill outside a city costs 5 food. Also those workers don't work for free, costs 1 gold each turn to pay their wages. Etc.

Imo there are quite some ways to balance it without just removing the possibility fully.

So what about this.  Wood gathering buildings that are within cities practice responsible foresting techniques and the resource supplies wood forever.  The type that pioneers can build wipes out a square of forest after a set amount of time.  So, you either need to send a new pioneer to harvest a new square of forest or you need to cast a create forest spell. 

Maybe the other option is something like Vandenburg said.  There are costs associated with the pioneer actually building a structure beyond just the cost of the pioneer itself so make it cost a lot to build a wood producing structure, or, make it cost quite a few pioneers going out to that spot.  Maybe nothing is produced until 15 pioneers have been sent to the site and that makes it more balanced.

I just hate spending city tiles on a patch of forest.