<Exposure time and time in the washout are the two most critical factors>
Not untrue, but any newbie may completely misunderstand the underlying concepts.
Exposure time is also a factor of how close the lamps are and how much the light diffuses through the unexposed photopolymer. The krene cover sheet in open face vacuum frames is an additional aid in diffusion, not present in glass-and-foam contact systems. This is why commercial units use close banks of UV tubes and not point-light sources. Diffusion is necessary to build a strong body beneath the printing face.
Then there's the removal of unexposed photopolymer. It does not dissolve just because it is in water. There must be physical action: brush or pile bristles, forced jet of water, ultrasonic wave. The upper softened layers must be removed before the lower layers can be softened by exposure to water (and water temperature is a definite factor, varying with different plate materials). The longer that takes, the more the printing face is subject to damage. A large plate and a small brush will require very vigorous action to remove the non-image photopolymer, and image details will be damaged, especially around the edges of the image (nothing more frustrating than having to remake a plate because one #%@! period or serif washed off). After using many kinds of stencil, shoe, fabric and pad brushes against the plate, I settled on filling a photo tray with actual photopolymer brushes (4x6 units from Gene Becker) and then making a magnetic plate holder to move the plate against the brushes in a warm bath, copying machine action, and gaining far more control and repeatability
I stress this after 25 years of hand development of photopolymer plates in my shop, and 20 by machine at various other places. Only had a machine in my own shop in the last 6 years. Finer typographically-detailed plates are possible by machine-washout than anything done by hand, though I still do the odd small job by hand.
If your concern is coarse artwork or display type, do what you want. Linoleum cut detail is easily matched by any DIY photopolymer methods. Text typographic detail really requires better control.
--Eric Holub, SF