having looked at both plastics(I use the term as generic and not in any sense derogatory) and composites, I rejected both for my deck. The following issues came up:

plastic only: Boards are soft-especially in hot weather. Need rafters closely spaced. No more than 12" apart even with boards perpendicular to beams. I had a diagonal board layout with 16" spacing making this impractical. If a deck does not use diagonal boards, use extra cross bracing to protect it from twisting motions in strong winds(newer building codes). Plastics have a high degree of expansion with temperature and so will expand and contract more than the underlying frame. This may affect the esthetics and is something you should be aware of. This is why vinyl siding gets wavy. Use of lighter color surface best to minimize color fading issues.

Composites: Much stronger than straight plastics. These contain essentially a mix of fine cardboard particles and plastics. Newer ones wrap solid plastic around outer surface for longer life. This is because mildew and fungi love to eat out the cardboard. Trex has had to settle numerous lawsuits from people whose decks disintegrated as the deck was transformed into a spongy plastic with all the cardboard particles eaten out. While these decks are strong and low maintenance, it is critical that they be thoroughly cleaned at least annually to keep mildew from getting established. How well the cardboard is protected is the key concern in a composite deck. In pressure treated wood, copper in the pressure treatment kills mildew. My old deck was cedar(untreated wood) and rotted through completely from mildew and fungal damage within 20 years. I could press my thumb through the tops of 4x4 rail posts. Deck boards disintegrated to the point of failure.

As an aside, the cardboard outer layer in modern drywall is why houses can have mold problems. Old plaster walls were far more resistant.