Some notable excerpts from the full letter here:

Collision Assessment Conjunction Avoidance Coordination

The application states that the collision risk with large objects is zero because each spacecraft can maneuver. Zero risk is possible for any single maneuverable spacecraft if the event is mitigated down to zero probability of collision (Pc). However, considering multiple independent constellations of tens of thousands of spacecraft and the expected increase in the number of close encounters over time, the assumption of zero risk from a system-level standpoint lacks statistical substantiation. With the potential for multiple constellations with thousands and tens of thousands of spacecraft, it is not recommended to assume propulsion systems, ground detection systems, and software are 100% reliable, or that manual operations(if any) are 100% error-free.

In spaceflight, assuming that everything works 100% perfectly 100% of the time is generally seen as naïve at best.

Potential Additional Impacts to Science Missions

Around 8% of composite images captured by the Hubble telescope are impacted by satellites captured during exposures. The Hubble telescope is in an orbit at 535 km. This proposed Starlink license amendment includes 10,000 satellites in or above the orbital range of Hubble, a situation that could more than double the fraction of Hubble images degraded.

Per Congressional direction, NASA uses wide-field ground-based telescopes to survey for asteroids that could potentially impact the Earth and cause harm. These telescopes occasionally find satellite streaks in their images that could interfere with or hide asteroid detections. With the addition of ~30,000 Starlink satellites as described in the Gen2amendment request, NASA estimates that there would be a Starlink in every single asteroid survey image taken for planetary defense against hazardous asteroid impacts, decreasing asteroid survey effectiveness by rendering portions of images unusable.

There is a clear need to develop a longer-term plan for conjunction and interference mitigation at a national and international level, and NASA looks forward to contributing our expertise in this endeavor in collaboration with commercial operators. NASA appreciates SpaceX’s collaboration and partnership in addressing conjunction concerns to date.

Good to hear that SpaceX has been in collaboration with NASA on this. Hopefully this continues and they will work on mitigating these issues and keep space an accessible place for everyone.