I think that you're underestimating the amount of damage Be'lakor takes from small arms without his defences of Shrouding and Snap Shots. T5 4+ W4, even with his 2+ cover save, is actually rather fragile for that price. Once he gets his defences up, sure, he's crazy tough to anything that's not a specialist shooting unit that ignores some rules (such as Broadsides with PENchip and C&C node/Markerlights). In this, I'd say he's pretty equitable to a Solitaire Autarch build, except with more support functions.
I'm trying not to underestimate how much damage he'd take. A squad of Sternguard all rapid firing (with bubblewrapping Be'lakor and deploying with my back to LoS blocking cover, or something, I can't imagine the opponent would be able to get multiple units in rapid fire range--so 1 unit of Sternguard in rapid fire range is equivalent to 2 at long), an enemy would get 20 shots, which accounting for hitting, wounding, and my 4+ invuln, averages 2.22 wounds. Yes, painful, and yes, I can easily flub that within a statistical deviation and lose him outright--but I do also have the ability to reroll one save (Fatey, yay!) and will have deployed exclusively to protect him. Plus there's no guarantee my opponent will get the first turn, so it'd only be against half the games in this specific list that they'd get that alpha strike at all before I'm swooping... (Initiative seizing going both ways, after all, though still mucking up guarantees, I'd probably deploy defensively so no matter what)
And like you say, he already has his 2+, so Shrouding doesn't help /him/--just those around him. It's swooping that he really wants (or casting invisibility on himself, if there are to be targets he wants to hit/good enemy Skyfire).
A nice thing on the Tau side of things: Missilesides are fairly static, all told. I can use LoS blocking cover to my advantage. Unlike against Wyverns (can deploy out of range of one) or Thunderfire Cannons (multiple TFCs, the real Be'lakor bane before he's swooping), I can deploy out of line of sight of the Missilesides (and would ideally be able to find a nice hiding spot after my T1 movement, too). Plus Tau are rarely going to be able to deny Invisibility... (score another point to Taudar)
That's another point in the list - not sure if it really has the damage output to kill hordes. You can do a number on Marines and especially GK, Wolf Guard, Sternguard etc. since the list puts out a small number of high-strength AP2 combat attacks with all the rules you could want tacked on, but I think even a moderate horde of Boyz (my Ork list runs 90 and has plenty of points to spare on Battlewagons etc.) - in these instances, you'd need to be very careful as you can't send any one of your units into combat. I don't have the maths handy, but I'm guessing that even Be'lakor with Invisibility would struggle against 20 charging Slugga Boyz or Poisoned Hormagaunts.
Yup. Like I said, I'be started taking a large block of Outflanking Seekers, now. Against hordes, I'd let them start on the board, probably (if few significant long range templates on the enemy side), and give them Invisibility right off. Then let them go a-dicing...
I admit this list was partially born out of a need for a viable counter to seeing the 3-Knight list somebody brought to a tourney. I need a viable counter to that--and it's hard to make a list that can survive that /and/ it's most extreme opposite, the horde. Somewhere along the spectrum of "take all comers" capability in your lists, you need to make value judgments as to what threats are weighted higher... and here horses and fliers have been given a bitire of an easy route. But not wholly, with Fateweaver's shooting and the Seekers, now, so...
Also watch out for melee - Be'lakor's mean in a fight, and being 12" move you can usually pick your fights and avoid bad matchups (much like Mephiston used to be, that glorious bastard); but it only takes a bad roll for Warp Charge or an unlucky roll for Invisibility can leave him out in the cold against pretty much any dedicated melee unit.
Oh yeah, without Invisibility and/or Grimoire on Be'lakor (both, if he's trying to take on a Knight) I'm quite hesitant to charge him in. A 4+ invuln won't hold against most melee units... though a 2+ invuln from Grimoire (coupled with EW and his I8) might. I have to pick my battles, though, certainly: take out targets of opportunity--whether they be stranded or already weakened--don't expose myself to counter-assaults, etc. a favored tactic is for me to charge him into an ongoing assault as a tide-turner, polish off the combatants there (say, finish off the Dreadknight that's been stomping my Hounds, on a different list's game), then use the consolidation move to bubble-wrap him against nearby enemy assaults if possible.
W/r/t things like the failure of Invisibility or chance to perils: yes, that is a very real danger. This list plays with high-risk high-reward abilities, like Grimoire and relying on psychic defense buffs. To say nothing of Warp Storm. But that's the joy of Fateweaver, as a counterbalance to skew my luck against the high risk side of things. In a list like this, he makes damned sure to make back his points--he almost can't help it, when his reroll lets me pass Invisibility; when the Warp Storm shifts from a result of taking a 3D6 Daemonic Instability test to all my daemons get +1 to their Invuln; etc... to say nothing of laying down a 24" AP2 S4+D6 beam every turn.
Be'lakor is definitely a finesse character rather than a brute force one, with a fragile statline, all told, but the potential for such a layered defense... he's much like Fateweaver, in that regard. That honestly makes for what I like most about the list: it's incredibly high skillcap list (to the point where I'm frequently kicking myself after games about things I ought to have done) that has answers to many facets of the conventional meta. It has its glaring vulnerabilities--against fliers and Ignores Cover, for instance--so I'm constantly tweaking for more balance.
But still, glaring vulnerabilities or no, for once, I can rely on a unit surviving the enemy shooting phase in /most/ cases. Coming from a few years of seeing editions punish my melee CSM more and more... That's gratifying. I have mobility, I have defenses, I can take out hard targets--at the cost of high prices and lack of unit duplication. If I make a mistake or mis-judge risk, I can expect enemies to capitalize on that--and I'm punished very heavily for my errors (which I honestly like, really). But it's not a cookie-cutter sameunitspam list that plays in a point and click manner--as damned near opposite that as possible, in fact. And it's damned fun to play.
Edit: I realize this turned into a real wall-of-text, but hey--thanks. Your concerns are very valid ones, and I've had a lot of fun writing about this and, really, exploring out loud part of why I enjoy this list so much, as well as the chance to really delve into the list's theoretical component. I think you deserve a +rep for inspiring (and putting up with) my long-winded ramblings. Also, this written on my phone, I'm certain there are some weird autocorrects that I haven't caught...