Turn on strict duplicate rules and reps start finding ways around them, turn them off and the database turns into a mess again.
The short answer on block versus alert is to block on exact, high confidence matches, an identical email address, for example, and alert on everything looser, like a similar company name with a different domain. Blocking too broadly trains reps to fake a field just to get past the rule, which ends up creating worse data than the duplicate ever would have.
Why default duplicate rules are either too strict or too loose
Salesforce's standard duplicate rules ship with generic matching logic that rarely reflects how a specific team actually enters data. Reps might type a company name three different ways over a year. A lead source field picks up an extra suffix depending on which form it came through. Default rules either catch too much, blocking two legitimately different contacts who happen to share a similar name at the same company, or too little, missing an obvious duplicate because a phone number was formatted with a different number of digits.
How to build matching rules around how the team actually enters data
Pull a sample of real duplicate pairs already sitting in the org and look at exactly what differs between them, a missing area code, Inc versus Incorporated, an extra space nobody notices. Build matching criteria around those actual patterns instead of theoretical ones, fuzzy matching on company name, exact matching on email domain, normalized comparison on phone number. Separate matching rules by object and by how that object typically gets created, since a lead from a web form behaves differently than a contact a rep types in by hand after a call.
Setting rules to alert instead of block where it makes sense
Block only for near certain duplicates, an exact email match, or an exact phone number paired with an exact last name. Alert for everything probable but not certain, and let the person entering the data see the potential match and decide, since they often have context the system doesn't, this might genuinely be two different people who happen to work at the same company.
Blocking a genuine near miss teaches people to route around the rule entirely, which defeats the point of having one.
Cleaning up existing duplicates before turning rules on
Blocking a genuine near miss teaches people to route around the rule entirely. A duplicate rule that reps have learned to defeat is worse than no rule at all — it just looks like protection.
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