Regarding credit ratings, there is a lot of information out there but generally people who pay their bills on time will have a beacon score over 700 and have no issues getting a mortgage.
People who have a lot of debt or are maxed out on their credit cards will lose points for that. There is a perception that checking your credit rating reduces your credit score significantly but this is not true for people that have high scores. If their score is 725 for example it might reduce the score to 721 which doesn’t really impact your credit at all.
If the client has a low score due to slow payments or high balances it has more of an impact. For example, they could go from a score of 640 to 636. With scores in this range there will be less lenders that will approve you for a mortgage and losing 4 more points could make it harder.