This plugin is amazing! The problem with using the roblox weld thingy is when you select a large group of parts it tends to create multiple of the same welds and put them all over the place. This only creates one of each and stores them nicely within the first part selected. Great work! 
WeldConstraint’s lack of a C0 and C1 property makes them unviable when dynamically updating the offset between two welded parts. With the new WeldConstraint, you’d have to disable the weld, before changing the CFrame of the part, before re-enabling the weld, which would be very inconvenient (and your CFrame isn’t in object space). The old Weld can be really useful for animating movement while keeping parts welded (which has many advantages), like in this case
In the video above, the arrow wiggling is done by updating the weld’s C1 property. This ensures the arrow sticks to whatever object it hit, while also having that nice animation
I’ve tested it and your statement is just wrong. Welds work perfectly fine with :ScaleTo()…