In all honesty my opinion is underdeveloped and mostly based on taste, so don't expect any smoking guns. I'll try to list a few points off the top of my head.

• Tuta generally doesn't support existing standards such as OpenPGP, where Tuta has it's own, better, encryption. Problem is, standards are widely used and serve as a basis for a larger ecosystem whereas Tuta's solutions aren't. My understanding is that this means in most cases you lose out on the minimal encryption offered in transit by most providers in exchange for a small network of well-encrypted emails. As a fan of open ecosystems and personal choice, I would at least appreciate the option.

• Tuta's products are often lacking. Expected or QOL features often aren't just missing, but are 'coming soon' for years in the cases where they are released. Tuta's services often do not work with third-party applications, which can lock users into an ecosystem and restrict choice. Worst I've seen is their export features which is still significantly less convienient than their import features, after years of changes and improvements, which feels almost like a trapdoor of sorts.

• I have a general distaste for anything that attempts to market itself too strongly, or promotes impulse decisions, as these are monopolistic practises that I avoid mainstream solutions to avoid. Tuta makes plenty of claims and announcements, but often doesn't inform users of the benefits or usecases. Tuta also often offers periodic sales, and a discount based off purchasing longer subscriptions, which is the standard admittedly, but tends to put me off personally.

That all being said, don't take this post as a 'Tuta is horrible and you need to GTFO' statement, but as an invitation to make sure your eggs arent all in a single basket.

TL:DR Use your own email domain, because changing between provider addresses is a hell I wish upon nobody.