I don't really consider Egypt low tier, but here are the most common points I hear:
1. UA is inneffective at higher difficulties due to AI production level negating its effectiveness, and on lower difficulties it's useless as you can pretty much get whatever wonder you want without it.
2. UU is lack luster and is barely an improvement over the already useless and short lived base unit.
3. UB aids the enemy and gives them an even greater incentive to attack you on top of the additional wonders.
If you have rebbutals I can only give token counter arguements as I don't think Egypt is low tier, more low/mid tier.
1: I would argue against that. It's not gonna get you the GL often if you weren't already in a position to get it, yeah, but wonders are extremely useful - including National - and getting 20% off them is quite powerful. Means you can get NC done more quickly and start expanding if you're going wide (and not on deity, I do it on immortal and still get a lot of good city spots though), means you just have more time to spend on everything else. It's quite universal, as if you're playing for Diplomatic/Cultural/Scientific victories you'll build quite a lot of wonders, and if you're going domination you'll build a few, plus you can expand more quickly as the NWs are build more quickly.
2. It's actually a very powerful UU if you want to rush early. I've seen it done many times; by the time everyone else has some archers up, these UUs will rip through cities. You can take 1-2 nearby capitals, hopefully before anyone sees you, and then gives you a very solid base to expand from, including much more land as those AI don't steal it all. Harder to do in Deity, still good in immortal, and actually quite a nice synergy with the UB.
3: The UB is my favourite bit. Absolutely incredibly powerful. Basically it guarantees you'll get a religion up to Immortal, as with the increased happiness and no gold maintenance it's easy to build. If you're losing cities then it doesn't matter how good your UB is, you've lost the game already, so who cares if it's increased gold capture? If you forward settle an AI then rush-buy this, sure it may be bad. But cities are lost so rarely in winning games. It also works really well with the UU, as you can rush and take 1-2 capitals early, beeline NC - faster with the UA - and then quickly spam out a whole bunch of cities. You can afford to have less luxes as well, as this is like a free extra colosseum early game. And if you want to play tall, it helps with early gold problems, is a building that's used in a fair few strats anyway, and gives happiness to allow you either a quicker GA, or to expand more quickly in pop size. Definitely helps with unconvential strats as well, where you don't get that +1/2 happiness from each citizen in the cap.
Overall not an incredible, god-like civ, but I would certainly but them above average. They're powerful in a couple of ways, not just one (Polynesia) and have pretty decent synergy between their unique stuff. I would defintely say high mid tier or low high tier, they're much better than a lot of 'mid tier' civs on the lists I see floating around.
But hey, mostly just personal preference.