This is similar to Kombucha, see my prior post from 2017 here. Every species (and strains) have different impacts and I prefer to reduce the risk of adverse outcomes.
“Similarly to milk-derived kefir, the exact microbial composition of kombucha cannot be given because it varies. “A Review on Kombucha Tea—Microbiology, Composition, Fermentation, Beneficial Effects, Toxicity, and Tea Fungus 
Great good can come from Kefir, for example, the probiotic Lactobacillus Kefiri LKF01 (DSM 32079) LKEF. It was discovered after investigating dozens of different Kefirs. See this 2017 post on this probiotic.
- Lactobacillus kefiri (occasionally)
- Lactobacillus kefiranofaciens
- Leuconostoc mesenteroides
- Lactococcus lactis
- Liquorilactobacillus nagelii,
- Lentilactobacillus hilgardii/diolivorans
- Lacticaseibacillus casei/paracasei,
- Lactobacillus perolens,
- Lactobacillus parafarraginis,
- Lactobacillus diolivorans,
- Oenococcus oeni
- Lactobacillus acidophilus,
- Streptococcus thermophilus,
The grains used, the processes and environment, all contribute to variable outcomes.
I have a strong bias in wanting to know precisely what bacteria are being taken. “Fermented drinks are good” is gospel for many…. for me, it is randomness… some people will do good and some will not – often dependent on the brand, or how it was fermented….. I am risk adverse…
Kefir grains could come salted with specific bacteria (to increase the odds) — but I would expect that the provider would provide information on the strains and links to the literature for each.