Is forcing T-SQL query to use parallelism OK? It can be done by calling the trace flag 8649 in a query option.

OPTION (QUERYTRACEON 8649)

This trace flag causes the cost of the query to be 0, which will always be below the cost threshold for parallelism and therefore the query will be considered for a parallel plan.

Does attempting to force a T-SQL query to use parallelism in this way have any downsides?

sample tutorial how to force parallelism
It has one big downside that is stated at the end of that post:


  Even experts with decades of SQL Server experience and detailed
  internal knowledge will want to be careful with this trace flag.  I
  cannot recommend you use it directly in production unless advised by
  Microsoft


Interesting to use when testing but not really for production.