UHMWPE film and fiber are very different. UHMWPE film is in no way a substitute for a composite of UHMWPE fibers (like cuben).
Consider carbon as an analogy. Carbon fiber composites (including laminar composites like carbon fiber-containing Cubic Tech products) are extraordinarily strong per unit weight. Carbon fiber composites are the gold standard for high strength, high rigidity, low mass parts. Monolithic blocks of carbon or carbon films, though, are mechanically worthless. They have very low strength and could never be used for load-bearing parts.
There are two major reasons that composites (like Cuben laminates and carbon fiber parts) are strong: a network of fibers creates many interfaces across which a propagating crack or tear must jump, and fibers can be highly oriented (much more than films) so long molecules are aligned to best withstand forces the part will experience. A piece of film has no interfaces that would retard the propagation of a crack or tear, and, even in "oriented" films (like Mylar), the molecules are mostly arranged in a chaotic jumble. Many polymers are available as both fibers and films (polyester, nylon, Vectran, UHMWPE), and the film is always much more mechanically weak and fragile than a layer of fibers of the same weight.
Also, as others have mentioned already, making seams will be close to impossible with UHMWPE film. Any UHMWPE film will be impossible to bond. The important property is free surface energy. The more free surface energy a material has, the more effectively it can be bonded. Plastic films that bond well (like Kapton) have a free surface energy in the neighborhood of 50 dynes. Mylar (the cuben surface) can be more difficult to bond because it has lower free surface energy: about 43 dynes. The surface energy of UHMWPE is about 28 dynes. This is close to some varieties of Teflon, which are in the range of 18-25 dynes.
There are adhesives that are marketed for UHMWPE surfaces, but don't overlook the bond strength values. Compare those to urethane adhesives for other films (like Mylar). Bond strengths for UHMWPE surfaces improve from abysmal (negligible) to poor if you flame or plasma treat them first to oxidize them, but plasma treatment is not practical for MYOG and flame treatments can't be used for films (it will just melt them). Even if you pay for plasma treatment and exotic adhesives, the best achievable bond will be very weak.
If you find a biaxially oriented UHMWPE film, you won't be able to heat-seal it because the oriented molecules will curl when heated, causing disorientation, deformation, and extreme shrinkage of the material near the heated area. Mylar (which is biaxially oriented) has this problem, and for the same reason. Mylar itself is never heat-sealed. It is often laminated to polyethylene (LDPE), and the polyethylene surfaces of the Mylar laminate are heat-sealed at a temperature low enough to avoid shrinkage of the Mylar. If you find a non-oriented UHMWPE film, you can heat-seal it the same way that painter's plastic or Heatsheets can be heat sealed, but non-oriented UHMWPE film will be similar in strength to painter's plastic or Heatsheets, so I don't see a reason to use it.