What 5th edition is all about

Tanks. Armor.
The cheap and readily available kind, that you can spam the field with.

Reliable infantry, capable of dealing actual damage to multiple unit types.

Mobility.

Putting 12 firewarriors inside a devilfish seems like a great idea, then you realize it's costing you over 240 points per squad, you need to dismount to shoot, all your guns are strength 5, don't hit all that often, the unit itself needs another unit to dismount the opposition, one more unit to make your guns hit, and a third to draw fire.
After that, you realize your masterplan involves driving your tau towards the opponent's knives, and laugh out loud at your own stupidity.

Until warseer, BOLS, and dakka all tell you this is a great idea, and should be encouraged, that is. They call it 'the fish of fury.'

That fish, despite all its fury, will calmly move towards your marines, while laying down an impotent curtain of strength 5 shots against your smoked rhinos. In the next turn, you drive closer, slag the devilfish with a well-executed hatch meltagun, and watch as your predators eat the wimpy firewarriors that pop out.

Now you ask yourself what went wrong. After all, he was in his transport and everything.
Devilfish are very durable transports, equipped with a constant 4+ cover save, multiple strength 5 guns, and, and...

And they suck.

You can't shoot out of them.
They cost lots of points.
They're built to stand off against infantry.
Units carried have NO flexibility, are strictly anti-infantry, need expensive support units plus specialists to function, and are not meant to take or hold the initiative - at all.

Compare this to the rhino.

You can shoot your two important guns out of the hatch.
They're very cheap.
They're built for taking the initiative.
All units carried want to get closer to the enemy, but if the rhino is destroyed, they don't mind being bunkered up midfield with some cover, or 'stranded' close to the enemy.
The rhino and the unit carried is a self-contained weapon of war. It doesn't need specialists or support to function, because the mounted squad brings all the necessary tools - meltas, flamers, powerfists, mass plasma, grenades, etc.

More expensive transports are indeed more heavily armored than the rhino, but that means absolutely nothing versus melta, or even most serious anti-tank. Land raider or trukk - the meltagun doesn't care. If it hits, your vehicle is likely to die.

"But my mounted tactical squads cost 240 points, too! Aren't they just as bad as fish of fury?" No, because the tactical squad doesn't need hundreds of additional points spent on support to function.

Also, consider this.
Tau get devilfish for two unit selections. Bar three bad units, and two elite units with land raider transports, you get rhinos and razorbacks for every single infantry squad you can field.
That's a lot of potential armor. The cheap kind, that lets you shoot out of the hatch. The kind that's so cheap, it tends to be ignored halfway into the game, so it can be employed to annoy your opponent with tankshocks, rams, and stormbolter shots for the last two turns, or used to screen important stuff during your advance.

If things aren't mounted, they're still fairly fast and mobile.
Bikes, jump infantry, skimmers, other tanks, walkers, infiltrators, relentless infantry, deep strikers...

Space marines aren't dangerous and powerful because they're blessed with more gear, more rules, and higher stats than most armies. Points balance all these out.
They're dangerous and powerful because they're built for 5th edition, and take full advantage of the current ruleset.

Next up, some basic wisdom that many players disregard, or deem not useful, due to the way it's usually shared.

Ed: the formatting just don't want to work for this one.