I understand your viewpoint, and some of it I agree with. I am also not saying the outlaw table has to change necessary, but we need to keep it in mind, and now with the pirate buccaneer it really highlighted some problems.
A lot of Xenos gangs should in my mind not be outlaws. Sure they are outlaws to the imperium, but not to the other factions. In my mind with the void pirates, rogue traders (who are known to trade with xenos), xenos warbands and lots of space travel we are not just looking at imperial worlds for income. I would even go so far as to say for xenos most income does not come from those iperials, thus the outlaw status should have only a minor impact if at all on income generation. Unlike in necromunda in my mind most of the income is generated of world.
Sure the imperials will disagree, but rogue traders often use 1 or 2 xenos with the risk of a death sentence, why risc a death sentence when it would mean less income? Because it actually opens much more trade options and leads to bigger income, not to less. You will have a hard time convincing me that being outlawed leads to an increase in income unless your warband is huge.
I think our vision also differs a bit since you as you mention rarely see xenos warbands, while in my case we could play because a large part of us prefer the xenos and thus over half the warbands in the campaign are xenos.
"outlawed isn't a massive hindrance... it changes the play style, but in necromunda house gangs can go outlaw... and do, then thrive without the tax system hitting them constantly." True, which is why more experienced gangs sometimes even like it. And here I might be mistaken, but the major advantage comes when you get large gangs and some nice goodies on the rare item chart. We do not use the rare item chart in inquisimunda and Xenos are expensive and thus smaller then house gangs, and much smaller then experienced house gangs. At least in our campaign most starting xenos gangs had around 7-9 members (only exception was once a starting orcs & grot warband had 12 members), most house gangs have slightly more (our starting chaos gang had 13 members)
So they have the xenos disadvantage of few elite members (thus earlier rout, less easy to take objectives, less people to generate income, easier to stop by pinning) and lose one of the 2 big advantage (few people to pay taxes over, elite members).
Considering the number of secondary objectives that offer some cash reward that are currently under development (I get it isn't fully released yet but the V1 has gone out), cash isn't going to always be poor. True, but it means that you need to use it to get money and you get less xp and because xenos are expensive you still have less members to get the money with, so still less money then other outlaw gangs.
They are still full gangs, so have access to their heavies (one downside to actual outlaw gangs...), I'm not sure why outlawing that freebooter warband, or that group of elder slavers is indeed such an odd thing to do? Because those dark eldar slavers are not scavenging for scraps, as do outlaws by the rules, they get their income from their trade in commoragh where they are not outlaws. Those Tau probably are trading and working outside imperial worlds. The we are talking about void pirates etc, here. They are definitely outlaws, but it is their job, they are not a bunch of hobo's like most cults and can trade with many people, it is just not that much the imperium.
I say most need to be looked at, Tau buccaneers, but also eldar slavers. They will not sell their wares much to imperial worlds, they will always be outlawed, but it will not influence their income, they get the income because they raid the imperial village for slaves and sell it in commoragh, not because they search for food in the imperial city in the hope they do not starve to death...
Eldar pirates are not in the fluff???
WTH 2 out of 3 factions are most of the time described as raiders/ pirates...
Tau I am unsure about. But even if they are not pirates they would have small companies of strike forces to trade with other races. sure it is all the greater good this and that and no other factions excist, except kroot, tarellians (maybe nicassar I am unsure) are known to be mercenaries for the Tau. So that Kroot/ tarellian who is fighting the imperials would clearly be quite possible to get normal treatment in the Tau empire even if not really part of the greater good.