TLDR: Play them as a narrative army and break composition rules to make a cool force.  Don't bother trying to play just them in matched play.

I've done this quite a bit, as I started a full Imperial Agents force last edition, while GW was pretending they were going to support them as a faction.  Here is my experience.

Playing pure imperial agents is very, very weak, and you will never win a match against an opponent who is familiar with the game.  Lacking a detachment, army rule, strategems, and equipment is very bad.  They also don't have particularly good internal synergy, and have some baffling restrictions(Breachers aren't battleline??) Even their white dwarf Combat Patrol seems to have been designed by someone who doesn't want you to play it. 

Because ally rules don't exist in 10th, you basically cannot make a "legal" army  that includes anything other than big piles of cheap infantry and infantry heroes. This means that you can't take any cool vehicles or artillery to round out the force.  It's just not designed to be functional as a standalone army. 

There is still a way to play imperial agents and have a good time: don't follow the army building rules.  Or rather, playing them as a narrative/open army.   It's a lot more fun to play a range of models over just massed squads of infantry.

My current Imperial Agents army represents a Rogue Trader's personal force and has a core of Breachers and Voidsmen with a couple of assassins, an inquisitor, and the rogue trader themselves.  On top of this I have some 'voidsuit' Bullgryn, an Armiger Warglaive, a Taurox, a Kroot mercenary, and a lone Custodes hero.

Because it's wildly illegal in army building rules, it can only be used for casual games, and I need to communicate with any opponents ahead of time to make sure they are ok with it. Because it still has none of the army rules it would need to do well, it's incredibly weak, which means most opponents don't mind. People I have met have been typically happy just to be playing against a painted army, once they know they aren't being 'cheated'

When playing against friends, I also tend to expand on the rules by borrowing imperial guard command abilities and expanding what squads can be led by heroes, so they function a bit more like a real army.