Karadjgne above makes good points. In addition, recognize that the USB designers specifically say that for Gen2 (10 Gb/s max) devices you really should be using the Type C connector and socket, because the older Type A system is not as good at high data rates and may NOT deliver the max capability of the Gen 2 system. So simply due to changing to a Type A connector on your SSD adapter may be a limiting factor.
Next limit is the SATA III (now more properly, SATA 6 Gb/s) interface. That is the upper limit of what that communication system can do.
Next limit is the SSD itself. Not many now can get to 6 Gb/s data transfer rate, although a few can. Do your know the spec for max SUSTAINED data transfer rate for your SSD (not peak rate).
It's not clear from your post why you say the unit you have is unreliable. Is it a matter of sometimes just does not work? Or are you measuring data transfer rates and find those to be variable? Or, are you just not getting the very high rates you expected?