https://www.reddit.com/r/(...)any_says_ukraine_to/The Leo 1 was designed in the era before composite armour was developed and when HEAT was king. At that time, there was no thickness of RHA that could stop a HEAT round, so the Leo 1 didn't try.
Leo 1 mounts enough armour to be small arms and autocannon proof, and artillery shell splinter proof, but not enough to resist dedicated anti-tank weapons.
Instead, it was designed to be a tank race car, with absolutely insane mobility. Furthermore, it fitted the incredible L7 105mm rifled gun, with HEAT, HESH, HE, canister, and APFSDS ammunition. It has 2-axis mirror gun stabilization, laser rangefinder, dynamic lead, and thermal sights - and the FCS is equal to or better than most Abrams.
It's a "shoot and scoot" tank, intended to pop up hull-down on a crest, fire a couple of rounds off, then back off the crest, jockey around, and pop up someplace else.
Your armour thickness is irrelevant if you don't get hit.
It will absolutely murder T-54/55, T-62, and any flavour of BMP or BTR. On later model T-72 and T-80 it will struggle somewhat on the frontal arc, depending on which flavour APFSDS it is carrying and if the target has (functional) ERA and/or later armour arrays. It will work just fine as an infantry assault HE and MG slinger especially if the combat team commander has his shit together and has good suppressing fire down, and it can close distance like a motherfucker (and it can reverse a hell of a lot faster than 4 km/h).
It is absolutely vulnerable to modern ATGMs. For command-steer missiles like SAGGER/SPIGOT it can theoretically dash to cover, but that is super terrain dependent and cannot be counted on. Something like Javelin would just outright kill it.
It cannot take tank HEAT/APFSDS hits on the frontal arc like Leo2 and Abrams can, and it doesn't have the pure anti-armour punch of either of the 120mm tanks, but it can still hit movers 4km out, and the thermal makes it hard to hide from. It's a great tank in the defense, assuming it has hull-down terrain to work with, and it can be a hell of a pursuit/recce vehicle so long as it isn't dealing with ATGMs.
A squadron of Leo 1 that sneaks through a gap or follows up a breach, running amok in the rear, is a very real threat.
================================
In a nutshell, the three characteristics of a tank are firepower, mobility, and protection, and firepower can be broken down into accuracy and penetration.
In WW2, all armour was some form of solid steel, usually some form of Rolled Homogeneous Armour (RHA) which is steel plates that have been squished between rollers to make it denser.
There was a (literal) arms race where someone would add more armour, then someone would counter it with a bigger gun, then someone would counter with more armour, and so on, and so on.
Then someone invented a new type of ammunition called HEAT - High Explosive Anti Tank. HEAT is kinda like a gun that shoots another gun that goes off when it hits the target - so it doesn't care about range (where solid shot loses penetration with range as the shell slows down) and because it is always point-blank, it has very, very high penetration. So much so that it requires so much steel to stop it that a tank with enough armour to protect against HEAT would be too heavy to move.
So in the West, we got a generation of tanks that abandoned armour (protection) for mobility and accuracy. The idea was to have a fast tank with a computer-controlled fire control system (FCS) that would see the other guy first and hit him with the first round, then scoot out of the way so his friends couldn't hit you back.
Tied into this was the development of thermal imaging, which can see in the dark and see through most camouflage.
The epitome of this style of tanks was the Leopard 1 - just enough armour to be safe against infantry and non-direct-hit artillery, but don't even try to protect against HEAT. Use the weight savings to carry a big ass gun, a really powerful motor, and the computers and optics so that you can hit a moving target 3km away while driving at 60 km/h cross country yourself.
Then someone discovered that you could beat HEAT with a layer-cake armour array made up of steel plates, ceramic plates, and rubber slabs. This was heavy and bulky, but it was HEAT-proof (ish). So the next generation of tanks (Leopard 2 and Abrams) kept the mobility and firepower of the Leo 1, but added the new protection from the new composite arrays. And then they added an even bigger gun