So you are standing in-front of 3 alternatives:
Obviously, building an electric bike is not for everyone, so let's concentrate on options number 2 and 3.
When buying an off-the-shelf e-bike, you get the convenience of having a turn-key product. Also, you will probably have a decent warranty and a place where you can go to if you need to do tune-ups or fix/replace some parts (Unless you delivered it from China or bought it from a dubious seller)
Moreover, an off-the-shelf electric bike is a "complete" bike, where the electric parts and the bicycle parts are integrated together with some taught behind (at least that what you get when you are buying an above average electric bike). It can be quite challenging to integrate the parts together so they will work synchronously (for example, adapting the motor power and your muscle power so you will be able to pedal even at high speeds) and it's something that you don't always can get from an electric bike kit.
Some electric bikes manufactures also use monocoque frames which houses part of the electric components inside the frame. By doing that, a better aerodynamics behavior and a slick design can be achieved. Obviously, you can't get a conversion kit which houses the battery inside the bike frame...
On the other hand, this integrated and complete electric bike could also be a problem because you are sometimes restricted to use parts only from a very specific vendor, you sometimes won't have the option to do upgrades and you also depends on the manufacture to stay in business in order for these parts to stay available (and in the case of such young industry, things can be quite unpredictable over time.)
A good rule-of-thumb is to always ask your self the question: Do I really need an electric bike kit, or can I get everything I'm looking for out of a off-the-shelf electric bike and at an acceptable price?
If you have a bike that you love and you are already accustomed to and also rides well and is in good condition, an electric bike conversion kit can be a great option. It will usually cost less then purchasing an off-the-shelf e-bike purely for the fact that you already own the bicycle. The converted bike would feel very similar to your unconverted bike but with a lot of added power.
With the electric bike kit you have a place to play. You can do a some refinement which aren't always available with an off-the-shelf e-bike. You can choose what kind of motor (speed) and battery (range) you want, aim for a specific weight and order a refined kit which answer your requirements. You'll end up with a bike that is "more you".
This is a good place to mention that some conversion kit manufactures offer their consumers to choose between different motor sizes, battery types, assistance methods, and other type of selections while other manufactures are more rigid. Pay attention to this if you think that you would want to do a future refinement to your kit or if you want more freedom of choice to begin with.
When buying an electric bike kit you get to choose the bike you want to convert from an unlimited choices of bicycle types. This is one of the best advantages of buying a kit. And for the type of bicycles which are not so popular among the electric bicycle landscape (e.g electric tandems, electric recumbents), buying a conversion kit and convert your own bike might be the only way.
But, installing an electric bike kit can deter a lot of people. Most of the time it is pretty straightforwards, but not always. There could be some things which don’t quite fit together, and making some compromises and thinking would be necessary. However, as opposed to a complete DIY electric bike project, you are not required to be a bicycle and motor expert or even a rookie expert in order to install an electric bike kit.
Another bonus of converting your bicycle by yourself is that you'll gain an experience for maintaining and repairing it. And after installing your first kit, it would be easier to do a second project or move the installed kit to another bicycle.
Nevertheless, if you're not at all mechanically and electrically minded, buying an off-the-shelf electric bicycle may be the right way to go. A kit is not for everyone.
From a safety point of view, you have to be sure that the electric bike kit is suitable to your bike. You have to be sure that you are not putting too much pressure on the bike which can cause it to brake (for example, in the case of high power front hub motor, the front fork should be made of steel and not aluminum.). In the off-the-shelf case, you have the assurance that the manufacture took care of the safety consideration (or at least you hope so.)
Also, performance and especially efficiency is limited. A bicycle should be designed from the ground up to be electric, there are a lot of things to consider when matching electric components to the bicycle components which is needed to be taken care of in the design phase by a bicycle expert.
Performance tests were done on a specific per of bike(s), and your bike could behave differently with the same conversion kit.