I would caution against using the FP in place of a workshop. The 2 day collect means a long dry haul between drinks of water.
Other things to consider are that from chapter to chapter your needs will shift. I don't know of anyone post ch 5 who cares about the coin & culture the FP offers. They are bonuses, not the main attraction. In my ch 4 beta city the coin & culture are more valuable to me but by ch 5 coin won't be an issue (I'll most likely have more than I need). The pop will be welcome thru ch 5 but in my ch 8 city I'm deleting magic residences because the upgraded housing offers a huge increase. Rune shards are helpful to craft runes or even craft AW KP in the MA.
My point is that what is most valuable to you now most likely won't be in subsequent chapters.
Thanks for the points. Below are my thoughts.
(1) The 2-day collection is indeed long. I have 3 - 4 workshops, though, so I thought they would be able to fill the dry spell.
(2) I understand that the main attraction of the FP is the feeding effect. Everything else (pop, culture, coin, supply, rune shard, and either coin rain or portal profit) is at best secondary. However, when compared to other production buildings, the produce from the FP may not be so insignificant.
In Chapter 5 (and I am human), a Level 15 workshop (3x4) produces 13755 supplies per day (four 3-hour runs and one 9-hour run) under 150% culture.
A Chapter 5, Stage 10 FP (4x4) produces 11520 supplies per day.
Therefore, we can already see that the FP does not underperform too badly compared to a workshop.
And actually, this relation reverses when we compare them on a per-tile basis.
The workshop, after accounting for the pop and culture it consumes, effectively occupies 26.86 tiles, so each tile produces 512 supplies per day.
The FP, after accounting for the pop and culture it provides, effectively occupies 3.6 tiles, so each tile produces 3200 supplies per day.
Of course, there are other factors that can turn the comparison in favor of the workshop. For instance, PP spells can be used on a workshop, and EE spells can further increase the output of the workshop. A magic residence would reduce the value of the pop provided by the FP. Also, the short production cycle of the workshop helps clear more repeated quests and thereby earn more rewards. However, then the above calculation also leaves out all the other fringe benefits of the FP (coin, rune shard, and instants).
In addition, the calculation can be affected heavily by which chapter one is in. For Chapter 5 human, the last residence technology (superior residence) was in early Chapter 3, so the residences are in a sense a bit outdated, which increases the value of the pop from the FP. In contrast, for someone who has just opened a new residence technology, the value of the pop from the FP might be smaller.
My point is that depending on each person's situation, the produce from the FP may not be so negligible.