we have been making 100G packet capture systems for 5 years now ( fmad.io ). In the early days vendor qualified transceivers really do make a difference, its 25Gbps signaling per differential pair which is anything but easy. Back then (4-5Y ago) the cheap QSFP28 vendors had some really marginal parts... we had to tune the fpga alot to get the QSFP28s to to work correctly, and even then some just wouldnt work at all / have alot of errors.

If your using latests Finisar or Avago level transceivers should be fine, currently (last 12 months) the cheap transceivers dont need any tuning too. Guess depends if your using old HW / old transceivers or new HW with new transceivers. 

Aaron