No uSD and SD isn't going to be as reliable as internal SSD. Not only the flash in the SSD will be of higher quality, the internal data protection scheme on SSD will also be a lot more sophisticated.
And on to uSD capacity, as you go higher on uSD, you should notice the speed rating also gets slower. This is because in order to get more capacity into such a smaller package, manufacture are using more bit/cell in the flash. All high capacity uSD is in TLC today while lower capacity will still remind in MLC, which is not only faster but more reliable. Large capacity uSD and reliability are properties that against each other.
Personally, I won't use uSD as long term data storage for important data.