It will only work if you add the following parameter:
OldGuids=true
And the query will work as follows.
let
Source = MySQL.Database("192.168.1.244", "crm_em", [ReturnSingleDatabase=true, OldGuids=true]),
crm_em_opportunities = Source{[Schema="crm_em",Item="opportunities"]}[Data]
in
crm_em_opportunities