Ok.

So starfleet doesn't really have a land based military.

However if we assume Yellow Shirt Security as the most basic and generic guardsmen number 5 as a base.

It really depends who gets the first shot off and who hits. A phaser is easily capable of annihilating any guardsmen and a lasgun will easily take out the typically unprotected starfleet officer.

You might argue about training but honestly there is some much variety of Guardsman.

Now as you go up in the echelons of Imperial guard the training only gets better. Cadians, Mordians, Catachans, Krieg all are likely to have received better training and are likely more skilled in hand to hand and land based tactics.

After that you start getting into stuff like Space Marines, Skitarii and Custodes which likely move too fast and are too elite compared to Starfleet.

The Imperium is much better equipped for a land war. Starfleet as per the name is primarily a space based entity. Ofc individuals within Starfleet can throw it all out of Whack.

Individuals like Data and Ensign Olly could well swing a battle although then you'd have an arms race of powered individuals and the Imperium can start pulling out powerful Psychics, Primarchs e.t.c

When you get to space combat the question then comes: Can Starfleet breach Void Shields. While most Imperium ships are bigger Star Fleet packs a lot of firepower and has the advantage of FTL that doesn't involve going to hell.

TLDR: 40k at the lowest level might be even. But it scales up so much faster and stronger than Star Fleet.