 If too much is pruned, there is a risk cutting out interesting nodes.Chess programs differ in terms of how and what types of pruning and extension rules are included as well as in the evaluation function.Some programs are believed to be more selective than others (for example Deep Blue was known to be less selective than most commercial programs because they could afford to do more complete full width searches), but all have a base full width search as a foundation and all have some selective components (Q-search, pruning/extensions).