Short answer: yes, a well-built recycled plastic park bench is generally as sturdy as a regular wooden one, and in several real-world ways it is actually tougher. I have seen both fail, but they fail differently.
A quality recycled plastic bench made from dense HDPE or similar lumber can easily support the same loads as a standard wooden bench. Many commercial models are rated for 750 to 1,000 pounds or more. The difference is feel. Plastic lumber can flex a little under weight, while hardwood feels more rigid. That flex is not weakness; it helps the material absorb impact without cracking. Cheap plastic benches, especially thin hollow profiles, can feel bouncy, so look for solid or reinforced construction, a metal frame, or commercial-grade thickness.
Regular wooden benches, especially hardwoods like teak, oak, or jarrah, are strong and stiff. But wood is organic. It rots, splinters, warps, and attracts insects. A wooden bench may start out feeling rock-solid, then develop soft spots around joints or ground contact within a few seasons if not sealed and maintained. Recycled plastic does not rot, does not need staining, and will not splinter. It can get surface scratches and may fade, but structurally it often outlasts wood.
The real question is not just which is sturdier, but sturdy for how long with what maintenance. If you want a bench that stays strong with almost no upkeep, recycled plastic is a smart choice. If you prefer the natural look and do not mind annual sealing or occasional repairs, hardwood is still excellent.
Bottom line: buy by spec, not by category. Check the weight rating, material thickness, and whether it is rated for commercial use. A well-made recycled plastic park bench will stand up to daily use every bit as well as a regular wooden bench, and often longer.