I'd say it depends on the engineered wood, the finish and who uses the bathroom. We have oak finished with osmo hard wax oil in a WC. We have noticed the finish is affected slightly in areas that regularly get wet and are left wet. For example where chinaware or skirting sits on the floor and traps water underneath.

We have kids and our master bathroom floor (tiled) is soaked virtually every night. I wouldn't recommend a wood floor in that situation. However we actually mop our wood floors (eg dinining room and hall) with a damp mop every few months and they have been fine, probably because they dry quickly.

I suspect some other finishes might be totally unaffected but Hardwax oil does look good and seems easier to repair/recoat when it wears (compared to say polyeurathane that may stand water better?).

I renovated our bathroom recently and used 'solid engineered bamboo' from one of the sheds. It comes with a 25 year guarantee and explicitly says it can be used in bathrooms. It's floated on a foam underlay over the original floorboards. All the planks are glued together as per the instructions.

I also made the countertop that covers the cistern and into which the basin is inset from the same material. But I gave that a few coats of varnish as well just to be sure.

It all seems to be holding up well so far. We're fairly light and 'respectful' users.

As far as I remember all the types of solid and engineered wood I looked at said not to use in bathrooms.

Greener than the solid Ads, if the solid needs replacing more frequently due to movement that allows water ingress.

I wasn't aware that bamboo is any more porous than any other untreated wood, ( or more correctly grass). It is certainly harder than many woods and I would have thought therefore it was less porous than wood. It's "green" credentials are considered excellent. Whatever you use however, it will need treating. Some engineered bamboo I've seen is a rich dark colour so perhaps it has undergone some sort of impregnation. There are many engineered variants some as you suggest are on multi-ply some are just a simple 3 ply. Personally I'd tend towards one on a birch ply backing, greener again than the tropical plys.

Bamboo is tough all right. I broke my best garden fork digging out bamboo roots. Bamboo is Asia's traditional scaffold tube, and bike frames have been made from it. It has the desirable combination of high strength and low density.

It is the low density that suggests me that bamboo in its natural state (300-400kg/m3) will be more porous than oak (740-930kg/m3). (Figures from http://www.realoakfloors.co.uk/wood_density_chart.php and http://www.engineeringtoolbox.com/wood-density-d_40.html)

But once the pores in both have been filled with a natural or synthetic water-repellent resin or wax, bamboo might well become the more water-resistant of the two materials.

owlman, since you and djh have both had positive experiences with bamboo flooring, I suspect engineered bamboo flooring may indeed be less porous than oak, though in its raw state bamboo would appear to be the more porous material.