I can give a real-life example from last year. I have the pay-version of the AGR Mastercard and I have full coverage on my personal vehicle with a $500 deductible. I used the AGR card for a one-day rental from Enterprise and declined all Enterprise coverages. While I had the vehicle, it was struck by another vehicle while my rental was parked. (At least, that's what I assume happened...the other vehicle left the scene without leaving a note). The damage was relatively minor, but of course I was on the hook for it as far as Enterprise was concerned.
I received a bill from Enterprise for approximately $850. Mastercard cut me a check for $500 (all I had to do was upload the Enterprise bill and my personal insurance policy to their website). Upon talking with my personal insurer, I determined that having them pay the $350 balance would have a detrimental effect on my future insurance premiums that would exceed the short-term benefit of having them pay the $350, though that's really a separate conversation. That said...
GEICO recommended Enterprise as being one of the less onerous of the rental car companies for charging loss of use fees. YMMV.
...I can't speak to what other rental car companies do as far as loss-of-use, but IME Enterprise should win no accolades. According to Enterprise, my damaged rental was in the repair shop for 12.8 hours (that seemed accurate). But here's how Enterprise calculated the loss of use fees. I had paid $29 (before taxes) for my one-day rental. So that's what Enterprise used as a baseline fee. Since 12.8 hours is effectively one business day, you might think that Enterprise would charge $29 for loss of use.
Enterprise calculated that since they charge for a full rental day once you pass the 4-hour mark, then they could have rented "my" car out multiple times while it was in the shop. Specifically, they could have rented it out 12.8/4 times. So they multiplied $29 by 12.8/4 for a total of $92.80 in loss of use charges.
That total isn't all that onerous, but you can do the math and figure that if you had a major incident requiring a car to be in the shop for, say, 40 hours, and you had rented that car for $50 a day, then all of a sudden you're looking at $500 in loss of use charges. So like I said, I'm not sure how other companies calculate it, but Enterprise sure isn't doing their customers any favors.
Although when I wrote Enterprise a letter asking them to show their reservation list for the day my car was in the shop, so that I could be satisfied that there was indeed enough demand that the car was needed for multiple rentals, they waived the loss of use fee entirely.