Cicero, Phil. 2.16: Quod autem idem maestitiam meam reprehendit, idem iocum, magno argumento est me in utroque fuisse moderatum.
Cicero, ND 1.1 (LCL 268): De qua tam variae sunt doctissimorum hominum tamque discrepantes sententiae, ut magno argumento esse debeat causam et principium philosophiae esse inscientiam, prudenterque Academicos a rebus incertis adsensionem cohibuisse.
In these two passages from Cicero, the clausal kernel is [x] magno argumento est [infinitive clause].
In the passage from Philippics, a quod clause serves as the subject. In the passage from De Natura Deorum, there is no explicit subject, but I believe the previous statement is meant to be the implicit subject.
I understand magno argumento as a dative of service argument, which is common with linking verbs.
But both passages then include an infinitive clause. The clause seems to contain the proposition for which the subject constitutes an argument.
My question is how to understand the syntax of this infinitive clause. Is it a substantive clause depending on argumento? The presence of a dative argument makes me disinclined to identify it as a predicate of esse.