Workers compensation, employer liability, and general liability answer different questions. Confusing them is how an injury claim lands on the owner's policy.
A $5,000,000 umbrella looks like it solves a $1,000,000 limit problem. Whether it does depends on follow-form language and the schedule of underlying policies.
A single requirement set applied to every vendor is either too strict for a landscaper or too loose for a roofer. Here is how the risk actually differs by trade.
The trades where you most need coverage are the trades a standard GL policy excludes. Contractors pollution and professional liability are the two policies that fill it.