Have to disagree. D21 (homeschooled) had nothing but online Spanish courses middle school through tenth grade. Took online AP Spanish in 10th and did very well on the AP exam. Tested into 300-level Advanced Spanish at our local state college for in-person dual credit for fall junior year, took that and another 300-level course this spring and has no trouble getting A’s. The professor last fall told her she was more mature and better able to handle the material than anyone else in the class (college juniors and seniors). She will spend next year there as a high school senior taking the highest level Spanish courses, the ones only the Spanish majors take. Again, her prep before testing into the advanced college courses consisted 100% of online learning. Exactly zero students at our local public high school take AP Spanish in 10th or go to the close state college for advanced Spanish instruction, even after all their middle school and high school in-person instruction.
I can say the same about two online science courses she took. She took online AP Chem and AP Bio using home lab kits and did just fine…I have a science degree from an Ivy and I can attest to the quality of those courses - they were difficult and they were taught extremely well.
Absolutely everything depends on how the course is taught. Are there multiple weekly live meetings? If not, are there 24/7 discussion boards with live appointments available and lots of TAs? Does the teacher/professor love his/her job and is he/she dedicated to the students’ learning? Are the lessons clear and well-organized with lots of options to turn to for extra help (videos, tutorials, appointments with the teacher, live help, study groups meeting both live via Skype/Zoom and online with discussion boards)?
There are bad online programs out there – just like there are bad in-person classes. And there are excellent online programs out there - just like there are excellent in-person classes.
General statements about which is better cannot be made. It all depends.