What kind of Perk you thinking of?
I played a lot of Rogue and Mercenary and Peasant Armies but I think Roguery is still to difficult to level because its real strength lies in early game to mid but it is way to time consuming and costly to level.
If you go to Bannerlord perks website and go to 1.5.6 (jeez, such a long time ago), the perk Partners In Crime on the 6th row presents the (unimplemented) description: "Bandit type troops never defect from your party (which currently is not present in the perk description anymore)." That perk means that no matter if your morale drops very low, no matter if you exceed your party size limit - *if you only have bandit troops in your party you can just keep recruiting bandits above those limitations because they'll never leave*. So you won't need a lot of clan tiers, you won't need stewardship or leadership, you would just be able to recruit a big blob of fairly weaker than normal units once you got the perk. All in good fun.
Now, I understand why it just hasn't been implemented yet. - For one, I think TW is big on morale and doesn't want that to be insignificant. That is why I am suggesting just limiting the perk to "recruiting bandits above party size limit does not cause them to defect" instead of no defections at all. A leader should still take good care of their party, naturally. Food should matter, morale should matter, etc - those are core game mechanics.
- Another thing that would make it stronger than it should be is the poor difference between low tier units and high tier units (armor is not effective enough at reducing damage still). So a lot of looters (3x the amount of a normal party) could possibly easily defeat mid to top tier troops atm. It's supposed to be a fun kind of perk, not an OP perk I'm sure.
- One more thing, the siege AI. Since archers on the wall aren't shooting the attackers as they are supposed to, I imagine you will get away scot free with having units with no shields and no armors in your party sieging a settlement.
- Finally, the last thing I'm thinking is the interaction with the "Veteran's Respect" leadership perk that allows us to turn bandits into regular troops. I mean, I can already see myself upgrade all the troops (lets say 500 looters) to imperial infantrymen with shields and better armors immediately after besieging a settlement (right before the attack), which sounds completely unintended gameplay. Then after winning the settlement I have proper troops defending it instead of the looters that I should instead have. So yeah, maybe some sort of limitation with that system is needed as well, because it sounds really abusable for sieges. Troops just wouldn't have time to defect from you upgrading them to no longer bandits if you did it right before a siege, so just make it so that they cannot be upgraded before a siege if it is too strong. If it's somehow not too strong it would still be a fun little idea and I wouldn't complain.
So yeah, there are some issues that need solving before implementing such a perk, but the main point is that TW gave the hope of a really cool perk at one point (very big bandit party size!) but it never happened yet, and it's been a lot of patches (the perk's idea was first revealed in 1.5.4 if you look at Bannerlord Perks again and it was called "Thieves Honor". So basically I've been waiting for this perk to come out for 7 patches now, which is a really long time. All I truly hope is that they didn't completely scrapped the idea, and they will come with some solutions instead. But every patch has me worried about it and roguery has some fairly useless perks for how hard it is to level, and this perk would give it the redemption it deserves...
About leveling roguery, I didn't level roguery yet since this perk is not out and I don't see the point. Yes it is difficult to level. Try using the prison escape system and help some lords escape, I remember that giving a proper amount of roguery experience, but I'm not sure how reliable it is long term. Also you can get daily experience I think if you have high morale (above 70?) and you have bandit units in your party, which again would be great if you could actually hire bandits over the party limit to gain more daily experience, so still waiting for that perk.