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.

Here is one more reason why you cannot blindly rely on DMV sys.dm_index_usage_stats. sys.dm_db_index_usage_stats does not get updated after an index is used for just the statistics associated with the indexed columns. Paul showed this in his link.

What sys.dm_index_usage_stats DMV does not tells you

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This article about index usage stats DMV

Moreover IMO I dont think by using DMV there is PERFECT way to find out when was table last used. I Belive using Profiler trace or Extended event trace would be more better option. Although profiler can cause load but server side trace is a good option.