I would say that what your repair shop is untrue, But since I tinker. It could be just a customer-facing person at the repair shop could've said that to prevent any potential issues.
Unless you get shoddy adapters. It should provide (See first page of this thread,
this one is recommended)
The only issues with using newer NVMe drives are:
1. More power consumption (Without Fix)
- Fix using NVMeFix kext with LiLu.
2. Lack of Hibernation support (On your particular Model)
- Work around exists (either change hibernatemode or flash new BootROM (HARD) )
3. You would have to make sure your machine has recent BootROM to support it
- Should be a non-issue if your machine already has High Sierra installed or Above prior to your original SSD dying
Otherwise you should see up to 1300MB/s-1400MB/s read/write, roughly 2X of your stock SSD.
Now there are some SSDs out there that are not NVMe (and is Sata Express) and has the compatible adapter with the Macbook Pro But those are either old, (not in production anymore and you would get one w/o warranty), expensive (for the performance involved) or both.
So either you get new NVMe (faster, more power used) or old OEM SSDs off eBay. (dubious quality/life expectency, lower power consumed)
I'd advise you not to get these:
(Fledging, OWC)
For recommendation of which SSD you get:
Personally Samsung 970 Evo Plus (or Pro) Sabrent Rocket 1TB (not 2TB)
But TBH. You need to do some research as which is the best to get in your use-case. Balance between what your system can do, vs how much storage vs your budget.
Also, for your model chances are that the bottleneck would be the 1300-1400MB/s limit from PCIe Gen2 x4 link to the ssd slot. So price your SSD accordingly. (You can look around in this forum also)