Vitamin B12 is an essential vitamin. The normal RDA (Recommended Dietary Allowance) for an over 19 years is 2.4mcg. Note that there is no upper limit set for B12 because overdosing does not cause toxicity. B12 is only contraindicated with those suffering from Leber's (hereditary optic nerve) disease.
Cancer risk results are mixed. There's some evidence suggesting that increased levels of B12 in blood might have a link to cancer and another set of data shows that low B12 levels has a link to cancer too.
Some studies showed that for age group 50 -70 years, who took over 55mcg B12 daily for more than 10 years showed an increased risk of lung cancer in men, but not in women.
But higher vitamin B12 intakes or serum concentrations were not associated with an increased risk of pancreatic cancer, breast cancer , esophageal cancer or gastric cancer. So you could see that these really are non conclusive unless some new data comes in.
So take B12 supplements moderately, you should be fine.
B vitamins are relatively safer to take as you excrete the excess through urine.
A lot of those adverse effects (liver issues, cancer, etc.) are associated with non-methylated B vitamins. Plus, some of us can't efficiently convert non-methylated forms of B vitamins into their active forms. So it's better to take a methylated B complex anyway. (do your research on this)
Another thing is, most of those B supplements contain super-high doses of certain vitamins. Unless you're fasting or you have a condition where you need them in super-high doses, that may not be necessary even though it's safer to do so. Personally, I've found that high-dose B vitamins (possibly B3 – not quite sure) affect my sleep negatively.