The Testudo was an armoured personnel carrier (APC) that was used by the Imperial Army during the Great Crusade and the Horus Heresy. The Testudo was the Chimera's predecessor and apparently looked no different than today's APCs. The reason why the Testudo was phased out by the Chimera was because the Testudo was lightly armed and armored whereas the Chimera have the same function as the Testudo but with improved defenses and armoring. Another disadvantage of the Testudo was that it used solid rubber tires, which were less dependable than metal tracks later used by the Chimera which is stupid because replacing a flat tire takes less time and is so much easier than replacing a destroyed track. Its disadvantage against the Chimera is also bizarre since the Testudo is one of the few Imperial vehicles that actually has sloped armor (If taken that its design is a modern APC) whereas the Chimera doesn't,
meaning that the Testudo has more chance of surviving a hit from a anti-tank weapon due to its sloped armor ricocheting the shot while the Chimera has to eat a direct hit meaning that the Testudo armour would have been more effective because it would offer a longer path of armor to an incoming hit. Another example of GeeDubs failing again. Although this could make somewhat sense if at some point during the crusade the Mechanicum discovered some alloy or way of producing armor that would work better at taking hits horizontally. Like some reactive material hardening whobbly.. stuff. (This is actually a real thing, look at modern tanks like the Leopard 2, flat sides all over. Modern composite armour has squares of ceramic in it that absorbs kinetic energy by shattering. If the armour is sloped the projectile will damage more of those ceramic sections without losing any extra energy, making it preferable to take that impact square on than at an angle.) That would explain the "boxiness" of late imperial vehicles. After all sloped armor decrease internal space and this is quite a problem in APCs especially. The vehicle's main advantage over most other military vehicles of the time was its speed, as the absence of thick armor or heavy metal tracks allowed the vehicle to reach impressive speeds.
The weird part of the Testudo is that despite being lightly armored, it somehow weighed at a hefty 40 tons which is as heavy as a modern Main Battle Tank. Whether the Testudo was lightly armored in comparison to its weight or it was another example of developer oversight doing a typo is unknown. The Testudo is also one of the few wheeled vehicles of the Imperium, the other being the Tauros.
The tracked vs wheeled vehicle debate is a very long and storied one in military design circles but as a rule of thumb, the heavier a military vehicle is, or the more it's expected to go on rough or soft terrain, the more it benefits from tracks. Similarly, tracked and wheeled vehicles have very different turning mechanisms, with tracked vehicles generally being more capable of turning in place than their wheeled counterparts. The most common contention in the tracks versus wheels debate is tactical mobility is inferior to strategic speed. A Chimera could go up and down hills, through muddy craters, and even turn in place, but a person arguing in favor of wheels says the ability to move the vehicle from one area to the next faster is worth more. As for sloped armour, modern munitions like the M829 would not ricochet off of armour it fails to penetrate; instead the projectile shatters due to the energy forces involved. So while sloped armour can defeat more primitive munitions with an efficiency that flatter armor cannot, now its primary advantage is increasing total armor thickness without using a larger (and thus more expensive) armour plate. Sloped armour does have a cost, however, namely in the form of interior space, which is why generally modern tanks no longer have sloped side or rear armour like some World War Two or early Cold War vehicles did. Similarly, the Testudo's forty-ton weight is comparable to modern vehicles in a similar role, particularly the T-15 Armada platform, though most of the heavier infantry fighting vehicles (IFV's) of today are tracked. Neither the Testudo nor the Chimera are particularly bad designs for an IFV; certainly there have been more hare-brained schematics proposed in real life and much weirder shit in the setting, but given the Imperium's frequent need to deploy on backwater planets or in poor terrain for wheeled vehicles one can see why the imperials prefer the Chimera. Additionally, the less powerful engines of the Imperium by the time of 40k means reproducing the Testudo would probably not be possible. Assuming an Admech would want to commit tech heresy, he would probably reason that stronger armor is easier to acquire than better propulsion (unless you work for the Gary Stus).
The Testudo was armed with an Autocannon mounted on a small turret, which was capable of turning a full 360 degrees, which allowed the vehicle to engage enemy units whether it was travelling towards them or fleeing. The Testudo's troop compartment was capable of carrying an entire infantry squad, up to twelve soldiers, yet lacked the Lasgun mounts featured on the Chimera. The troop compartment only had a single exit, a rear ramp wide enough to allow two soldiers to exit the vehicle side-by-side. The inside of the troop hold featured benches that ran the length of the hold, allowing the passengers to sit down. Near the front of the passenger hold were ammunition baskets that held the ammunition for the vehicle's turret-mounted Autocannon. It is unknown if the Testudo was able to be outfitted with other weapons or upgrades.