All points below refer to grade 5 titanium (e.g. 6Al-4V). Most all reputable manufacturers will be using this alloy for structural titanium. All points also assume the stock steel part is replaced by the same geometry, but in titanium. Obviously if someone makes a titanium part that's thinner than the steel part it's replacing, then that's a whole different scenario.
Strength:
In most all cases, titanium will be as strong or stronger than the steel it's replacing. There are certainly steel alloys and heat treatments that are stronger than grade 5, but it's unlikely they're being used on a stock bike anywhere.
Fatigue Life:
This is a minor concern. Depending on the design stresses, titanium will fail from fatigue earlier than steel will. But for most riders it won't be a concern. Unless you're riding A LOT and keep the titanium parts for a long time, you probably won't have any fatigue issues.
Hardness:
Again, in most cases, titanium will be similar in hardness to the stock steel it's replacing, so wear isn't a huge concern.
Corrosion:
Titanium is very inert and will not corrode unless in an extreme environment not found on a dirt bike. It will, however, accelerate galvanic corrosion of the material around it (aluminum, steel, etc.). This can be negated with adequate lube/anti-seize and/or coatings such as TiN and DLC.
Assembly/Seizing/Torque:
This is a big issue for titanium. You can't treat it like steel, you have to be much more careful. It likes to gall, particularly with itself (e.g. nut and bolt are titanium). The solution is to use a lot of anti-seize as others have mentioned. But when using anti-seize, the tightening torque changes substantially. The anti-sieze acts as a lubricant and for the same torque as a dry steel bolt, you'll end up with much higher bolt tension. That's bad. You'll need to account for that, and in general, reduce your torque by ~20%.
We have a handy torque spreadsheet available here if you'd like to play with some numbers:
https://luxonmx.com/resources/luxon-bolt-torque-calculator.xlsx
Stiffness:
This one will likely cause some argument... Large chunks of titanium replacing steel in the primary load-paths will flex more and MAY be noticeable to the rider. The vast majority of people won't know the difference unless they're told it's different (and there lies the problem, the placebo effect is strong). So picky riders and those that notice subtle changes may pick up on more flex from swingarm pivot bolts and axles. No one is picking up on the difference between titanium triple clamp bolt stiffness vs. steel. Again, the placebo effect is STRONG.
Summary:
Titanium replacement parts will be just fine for the vast majority of people so long as you understand the increased maintenance requirements (torque and anti-seize).
And finally, to address this:
Fun fact, Titanium is actually heavier than AL from a density standpoint. However, it's strength characteristics allow for it to be implemented in a way that makes it lighter than any other metal. You can run thinner walls on TI and still come out with a higher safety factor and less weight.
That's not typically true, it's highly dependent on the manufacturing process, alloys selected, and design criteria. The stiffness to weight ratios are nearly identical for most structural metals - steel, titanium, aluminum, magnesium, etc. And strength to weight ratios can be pretty close as well when selecting appropriate alloys. What this means is that for the same weight, a lower density material will be thicker. While this has minimal effect on tension and compresive stiffness/strength, it has an enormous effect on bending stiffness/strength. The lower density material being thicker (for the same weight) will almost always be stronger and stiffer. Additionally, depending on the manufacturing method, you are stuck with material that isn't optimal. For example, a machine part can not have sharp inside corners because an end-mill has a finite radius. And it needs to be stiff enough to cut the material. This results in a radius on the inside corners of parts that doens't "need" to be there for strength/stiffness, but does for manufacturing. So that material is "wasted" weight and it follows that a less dense material will be better suit that situation. Now if we're talking about 3D printed titanium or something, then that is less of a concern, but the bending stiffness point still applies.
So, for example, a steel titanium triple clamp will be less stiff and weaker than a titanium one. This works on down the line - ti will be weaker and less stiff than aluminum. Aluminum will be less stiff and weaker than magnesium. Magnesium, even AZ31B, is a special case, as it's a lot weaker than aluminum so it's not a good choice here typically. Plus there's the corrosion and manufacturing issues to be concerned with. This is why you primarily only see triple clamps made of aluminum.
Titanium shines in places where you're space limited, and bolts/axles are a great example. You can't replace a solid titanium bolt with aluminum and have it be stronger, there wasn't room to add any material. Similar for an axle, since you can't go to a larger diameter, you can only add material in the middle where it's not very effective in bending. (there's a whole thread here somewhere about aluminum axles and why that's not a good idea).
