Biggest items I can think of:
1] Moisture in the pipe will most likely kill regular cat5 transmission speeds, you'd need direct burial rated cable (www.syndat.com says direct burial cable exists as "763108 - Helix Category 5 Direct Burial - Black"). I've heard of telco lines run underground which are frequently ventilated or otherwise dried out to improve a degrading signal conditions, due to moisture contacting the cables. The direct burial cable is likely designed for this.
2] Electrical noise gets coupled from powerline, despite the cat5 shielding.
Remember that normal runs of cat 5 would be allowed to randomly wander away from the power cable, when it is run in, for example, an 8 foot tall wall or along 30 feet of crawlspace. Your 50 feet of 1" pipe will ensure that the cat 5 won't get very far away from that power wire, and this close proximity may reduce shielding effectiveness. Ask a pro cable installer about this coupling effect; and also ask about coupling at a cable television installer or supplier, if you dare! They know it can happen with shielded cables, and will wonder why you ask.
Your workshop implies motors, possibly brush type motors (dremels, hand drills). I think your condition (the forced mechanical proximity and "noisiest possible" neighbor) is the worst possible electrical environment for any data cable.
3] I'm in New Hampshire, being as it is presently mid-winter, frost is rampant. 50 feet is a long underground run, frost movement is very likely. If that pipe ever mechanically shifts, as in frost induced shifts, you might pinch the data cable into the power cable... AC line power conductors meet low voltage data conductors. Pow! Exploded NICs (at best).
I may be overly pessimistic, but don't forget that any water in the cable would *mechanically* degrade the shield: aluminum foil shield vs water means galvanic corrosion and thus shield degradation over time. How quickly? Don't know.
Also, the moisture in the pipe isn't exactly pure, the pipe isn't necessarily intact, the ground isn't necessarily unfreezable, etc., so there may be contaminants in the trapped liquid which would further corrode the individual wires themselves, once the jacket gets compromised.
To me, the end result is don't try it, unless you feel that the risks are acceptable. Your mileage may vary.
I'd consider RF wireless data connections, or possibly fiber optics in that pipe: nothing is conductive in the fiber cabes I've heard of (but you need to check them out first), thus without metals you eliminates the corrosion and short circuit angles I mentioned. Then, if water degrades the cables' optical properties or if the frost pinches it, simple data loss is all that will result.
That's my thirteen cents.