Rendering is basically a cooking process to concentrate proteins or fats from a mammalian meat source.
There’s nothing inherently wrong with rendered meat (“meat meals”) for dog food. It’s considered a good source of protein or fat. Commercial dog kibble needs to have some meat meal or similarly rendered and dried product in it in order for the kibble to be able to be formed. Kibble is a dry product, so using meals allows for formulation.
However, I prefer a food that also includes fresh meat.
This is because rendering is more processed, and I prefer to provide my dog with foods that are less processed.
Also, different cooking methods both remove or inactivate certain nutrients and make other nutrients more bioavailable. If both meat and meal are used, that increases the variety of nutrition and nutritional bioavailability in a food.
While rendered meat products have a higher protein content by weight, they are also over 30% less digestible than meat meals, have fewer vitamins, and increased oxidation. Meat meal is high in ash and protein, which can be balanced either with plant products or fresh meat. Fresh meat-contain products often are more palatable to dogs as well.
I’d like to use this as an opportunity to mention quality. We don’t hear much about actual ingredient quality. In dog food, the regulatory standards for quality are very low, so any given ingredient can meet regulatory standards but still not be something you want to feel your dog. At the same time, ingredients can be of such high quality that you might feel comfortable eating them yourself (note, however, than “human grade” is a separate issue and has separate regulatory hurdles to meet).
Personally, I want to work with a company that is honest and transparent so that I can feel comfortable with the level of ingredient quality as well as the information that is shared with me. We all have to decide for ourselves how we feel about a given company in this regard.
 Diallo, A, A Swali, J Lawson, B Streit, S Hill. Low temperature processed meat in extruded dry petfood: 2- Effect of commercial scale processing on nutritional and eating qualities. Manuscript. Department of Food Sciences, University of Nottingham. Poster, manuscript, and presentation at Pet Food Forum Europe, 2015 https://www.atavik.fr/wp-content/uploads/2018/09/pdf-etude-frais-deshydrate-fr-2018-update.pdf (scroll down for the complete paper in English)
 Raw, mechanically separated chicken meat and salmon protein hydrolysate as protein sources in extruded dog food: Effect on protein and amino acid digestibility; Tjernsbekk MT, TAuson AH, Kraugerus OF, Ahlstrom O. Animal Physiology and Animal Nutrition 2017. https://onlinelibrary.wiley.com/doi/abs/10.1111/jpn.12608
”What is in Pet Food.” AAFCO (Association of American Feed Control Officials). Last accessed 01 April 2021. https://www.aafco.org/Consumers/What-is-in-Pet-Food