I covered this topic before several times, including Why I Always Test Directly with Each DNA Company – DNA Saturdays February 6, 2021, ISOGG Autosomal SNP Comparison Chart – DNA Saturdays December 26, 2020, among other posts. Yet, I continue to see people in social media groups encouraging others to upload to MyHeritageDNA, FamilyTree DNA (FTDNA), LivingDNA. Here’s the amount of shared SNPs (markers) shown by ISOGG: https://isogg.org/wiki/Autosomal_SNP_comparison_chart. Once you realize how few markers are in common between not just different companies, but between different chip sets in the same company, it’s a bad deal. You will wind up missing a lot of true matches by going this route. You probably won’t miss close matches, but they will show as more distant than they really are. For parent/children or full sibling, not as big as a problem, but it could still lead one astray if the SNP comparisons are fairly small with a direct upload.
AncestryDNA and 23andMe don’t allow uploads from any company. I don’t blame them as there are large differences between the various chip sets with each company.
I haven’t uploaded to GEDmatch as I find their practices questionable and after a few data breaches and not acknowledging when they started letting Law Enforcement to use their services. If judges, prosecutors, and law enforcement were more knowledgeable about the limits of DNA. I don’t agree our Founding Fathers would allow police to dig through trash or pick up discarded DNA without a warrant if they had known about DNA. That’s not counting bad police who have no problem placing false evidence in someone’s trash. I have seen too many people – police, DNA experts, etc. who have no problem crossing a line because they think someone has gotten away with a crime. The first time you cross the line is difficult, but some are so sure that a person committed the crime without following other leads.
I believe nobody gets away with anything. If you don’t answer in this life, you will answer in the next life. It doesn’t matter if you don’t think there is a next life because one day you will find out it exists.