I read a book on data warehousing the other day. The author claimed reading from the transaction log had no impact on the source system. The author might have meant reading from the log doesn't have a significant impact.
Reading from the transaction log has overhead. Now whether that overhead matters or not depends on the workload and SLAs.
I simulated an OLTP workload against the HammerDB TPC-C standard database. Next I executed read queries against the log. The test showed reading from the log decreased database output by 20%. The bigger the transaction log, the more this will impact the system. The dips in output correlate with the execution of the log queries.
The red arrows point show the points in time when the transaction log was queried directly