Is Information in sys.dm_db_index_usage_stats reliable
No. 

The sys.dm_db_index_usage_stat view reflects only, at best, data since the last database startup (last instance startup, or last time the DB was brought online). Furthermore the entries may clear under memory pressure. It will give accurate positives (if a table has non-zero stats then it is used)) but may give false negatives (0 usage in stats may not reflect actual usage). Also there are many systems that use certain tables only once per week, once per month or even once per year.