Where I live, we have great fluctuations in humidity beyond 90% and perhaps 30-40% in Winter months. I seal all exposed MDF with thinned epoxy unless it is getting unbacked veneer. I have sealed some cabinets under veneer with epoxy, but this would mean that raw, "unbacked" veneer would have to be adhered with epoxy as well for a compatible bond. Because of this humidity changes, raw veneer can creep/ripple over time when using softer adhesives such as contact cement, so special, hard setting, veneer glue is recommended.
No matter how one approaches such things or great finishes, you always pay the full amount somewhere in the process and typically in the preparations. There is no shortcuts! I notice I have no friends when the blocks and sandpaper are in hand!