J&J is an adenovirus vector vaccine, which is barely less experimental than mRNA. AFAICT the only licensed adenovirus vector vaccines are for Ebola, and those have not been used very widely.
Earlier attempts to use adenovirus vectors for gene therapy had issues due to, wait for it, thrombocytopenia.
The the one and only COVID vaccine that uses more or less tried and true technology and looked excellent in trials is Novavax. (AIUI Novavax’s adjuvant is newish, fancy, and not necessarily that easy to scale, and it may well be responsible for the excellent performance.)
Earlier attempts to use adenovirus vectors for gene therapy had issues due to, wait for it, thrombocytopenia.
The the one and only COVID vaccine that uses more or less tried and true technology and looked excellent in trials is Novavax. (AIUI Novavax’s adjuvant is newish, fancy, and not necessarily that easy to scale, and it may well be responsible for the excellent performance.)