Absolutely there is added risk of injury with higher training volumes, the body can only be loaded with so much before it breaks. Players need to be conditioned up to the point of higher intensity, throwing in players who aren’t conditioned effectively is only asking for an injury. I know that’s why some on here object to young players getting thrown in too young as well. That’s where a good S&C coach is worth their weight in gold, they can identify the strengths and weaknesses, and tailor training so that all players are working to their capacity.
What league benefits from is a longer pre-season, which allows for a more tailored and structured pre-season with heavy loading and light loading phases. Reds/Australian Rugby have a competitively short pre-season, especially in 2019-2020 when the first game starts in early Feb.
And like you mention, the summer heat is going to throw an extra spanner into the world, and how that then flows into the early part of the season will be interesting to see.