HeLa cells are not more immortal than many other similar cells lines. They aren’t especially easy or difficult to passage or handle, either. The biggest thing they probably have is that many early studies were done with them, and because other researchers subsequently built upon those results, the same cells remained in use. So they’re not special, they just happend to be the first ones to be commonly used and thus also well-characterized.

As a side note, nowadays many HeLa cell lines have mutated and are fundamentally different from the original lines, so it is almost as if you worked with completely different cell lines if you use certain HeLa cells.

Indeed, HeLa cell have been reported to contaminate even some commercially available cell lines, and probably contaminate many cell lines where researchers do not even know they are contaminated. I do not know how extensive this problem is, though.

Most cell line providers have extremely high quality controls nowadays, so it is unlikely they have this problem any more. I think this problem is not quite as big as it perhaps sounds, anyway.