Presidential pardon power outstrips Lord Protector's?

I just noticed that Oliver Cromwell's power as Lord Protector of England explicitly excluded pardons for murder and treason by the constitution of that revolutionary government. The pardon power that the U.S. Constitution later vested in presidents, meanwhile, at least in the readings of the Supreme Court, has no such limit. Indeed, Gerald Ford effectively pardoned Nixon for treason, if you ask me. That's power.

Makes me wonder if the Supremes got this right. Cromwell's constitution must have been at the forefront of the Anglo-Protestant framers' minds, and so we're all told were the liabilities of unchecked power, for which Cromwell was the natural model under the scheme of representative government they were devising. Maybe the framers viewed the limits of the pardon (no pardon for murder, no pardon for treason) obvious and implied?

Postscript: No, Madison foresaw corrupt presidents pardoning treasonous henchlings, and this is why, he said, presidents must be subject to impeachment. Or so said some scholar I saw talking to Bill Moyers on TV.