It is not as strong as epoxy. On wood it is stronger than the wood so its strength is not an issue but on metal, epoxy provides a stronger bond.
It compares to other wood glues. Great claims have been made but tests seem to show about equal holding power. Polyurethane had better end grain gluing capability though because of better glue penetration.
Parts being glued with polyurethane needs to have a tight fit to be strongly joined. polyurethane is not a good gap filler. Although it is sometimes advertised as gap filling, tests show that strength is reduced when the glue line is thick. When setting it froths up and the froth, although gap filling, is not strong. Some formulations have filler that allows for some gap filling.
In very dry areas it requires moisture to set or it can set very slowly and not reach it's full potential.
Because it is sensitive to humidity in the air and wood, setting times can vary considerably.
It tends to foam and squeeze out. If not clamped the parts can be forced apart by the foam, weakening the joint. The foam gives the illusion of gap filling, but it is not strong. Foaming is greater when there is a lot of moisture. Some compositions foam more than others which are practically foam free such as the construction adhesives as PL Premium.
After curing it is very chemically inert and safe but the intermediate phases are toxic, irritating or carcinogenic. This means polyurethanes have to be handled carefully, kept off your hands and not breathed in too much. See the side box for the Wooden boat forum link which has a good discussion on the safety of PU glues. You can google for the MSDS of various brands to get more information.
Warnings include: Contains isocyanate containing polymers. Contact causes eye irritation. Prolonged or repeated skin exposure may cause allergic reaction, irritation and sensitization. Contact may stain skin. Do not allow eye contact. Avoid prolonged or repeated contact with skin.
Some brands and compositions are expensive.
Polyurethane has a limited shelf life, less than a year, and once opened can go off quickly if moisture gets in.
Polyurethane is difficult to clean off hands, (gloves are highly recommended). Acetone or lacquer thinner can be used to clean tools while still uncured.