People have been worried about the provision of Universae Ecclesiae n.19 and I thought it might be helpful to look at this more closely. First let us examine the text itself in Latin and in the English translation:
19. Christifideles celebrationem secundum formam extraordinariam postulantes, auxilium ne ferant neque nomen dent consociationibus, quae validitatem vel legitimitatem Sanctae Missae Sacrificii et Sacramentorum secundum formam ordinariam impugnent, vel Romano Pontifici, Universae Ecclesiae Pastori quoquo modo sint infensae.Here is another translation which is a little more slavishly accurate (among other things, recognising where quoquo modo is in the sentence):
19. The faithful who ask for the celebration of the forma extraordinaria must not in any way support or belong to groups which show themselves to be against the validity or legitimacy of the Holy Mass or the Sacraments celebrated in the forma ordinaria or against the Roman Pontiff as Supreme Pastor of the Universal Church.
The faithful who ask for the celebration according to the extraordinary form must not support or belong to groups which oppose the validity or legitimacy of the Holy Sacrifice of the Mass or of the Sacraments according to the ordinary form, or which are in any way hostile to the Roman Pontiff, the Pastor of the Universal Church.There are some who deny the validity of the Novus Ordo: for them Universae Ecclesiae will be irrelevant since they will not be terribly interested in the present liturgical legislation of the Holy See. The word which has worried more people is legitimitatem.
Purely as in incidental observation, I wish that the Latin text had used a happier expression: legitimitas is not listed in Lewis and Short, and even in an equivalent English sentence, would be barbarous nounificationalityness. Still, we can understand it easily: we are not allowed to impugn the legitimacy of the Novus Ordo. In other words, we can't say that it is illicit, unlawful, illegal to celebrate it. That is sensible: otherwise we would have to say that the above photo shows the Pope doing something unlawful. Some people might believe that, but again, I doubt whether any of them would be bothered too much by the legal force of Universae Ecclesiae; they could always get Pope Michael I to issue a condemnation of the document. For the rest of us, the clause should not present a problem.
UE n.19 does not say that we have to like the Novus Ordo, that we have to think it is better than or equal to the Mass of Ages in respect of beauty, tradition, or expression of doctrine. We can criticise it, we can even say that it is a "banal, on the spot product." We do not have to say that the Novus Ordo is the most beautiful thing this side of heaven; only that it is lawful to celebrate it.
On the subject lawful celebrations of Mass, here is another form that may be celebrated lawfully. Not only that, but on account of its venerable and ancient use, it must be maintained with due honour (UE n.6):