The highest grade, reliability wise, goes in industrial grade products; the next following grade goes in name-brand SSDs; the lower ones in mass-market memory cards. One day an employee at a famous NAND manufacturer said "That's not even worth of a C grade", on that day the USB flash drive was invented...
Reality, especially modern one (as in early 2010s+n), is obviously more complicated, all sorts of consumer products are made with all sorts of chips, with name brands and even specific models/price class being only loosely indicative of quality, specifications (such as density: SLC/MLC/TLC/QLC/PLC - how many bits crammed into each capacitor) not always available and in any case not contractual with respect to the model, counterfeiting and/or Flash Fraud (inflated capacity with address-overflow overwriting) still ubiquitous in certain segments, ...
eMMCs are indeed generally a tier above the common microSD in reliability (compensating for soldered vs replaceable?) and in performance (how many UHS-II cards and readers have you seen in person? Most eMMCs are designed for, and used in, 8-bit mode and are usually fast enough to benefit from it), never mind that your phone might be using a newer and faster technology like UFS for its internal storage!
That said, your specific example is unlikely to be coincidence; you're suspecting the contacts (which indeed I find to often go bad on the modern microSD+SIM tray designs), so do the cards work in an USB microSD reader? (Avoid micro/miniSD to SD adapters, their thickness-constrained design also loosens and goes bad at an appalling rate!)