Certainly the EO POV is that an RC has something lacking, but the implication is not necessarily having a valid baptism.
Fr. Matt, I like how you quickly write back answering questions that people bring up.
If you take the view that RC commuion is invalid and that one should never take it, you are in good company with Prof. Osipov. And even Abp. Elpidophoros is now openly saying that the EO position is no regular communion for hetorodox spouses. I am not sure whether he told L100 that the spouses may commune, but I don't feel that I care enough to find out for sure. If he did, he has now backtracked and abandoned that position.
Regarding the validity of heterodox sacraments, and particularly baptism, I read an MP theological article on a Russian website analyzing the theories expressed by Orthodox theologians. It had a range of views and in this regard reminded me of a similar EO theologian's article discussing the RC teaching on the Immaculate (sinless) Conception of Mary and EO views on that teaching. Namely, EO theologians don't emphasize Mary's "Immaculate Conception", but the EO article explained different views and reasons.
For instance, one idea is that EOs don't accept the RC idea of original sin being passed down biologically, and hence all births are "immaculate" from sin in the way that Mary's would be. ie. the RC belief is that God miraculously made Mary herself born without sin as an exception to the RC belief that all people are born genetically guilty of Adam's Original Sin. the RCs believe that this occurred so that Jesus Himself would be free from sin in His birth, coming from Her flesh. And so while we EOs do accept Mary's Immaculate Conception, the issue is moot for us, because everyone has an immaculate conception.
Another idea was that Mary always in her life remained personally free from the stain of sin. This idea is taught by the RCs and by some EOs. We sing that she is "most pure", etc.
Yet there has also been patristic speculation that Mary at some point sinned or may have sinned, like when Jesus' family called for Him to come home because he was preaching publicly and Jesus commented that whoever does the will of His Father is His Brother, Mother, and Sister. And the article may have speculated that even if Mary sinned, the issue is moot in terms of Jesus' own sanctity and sinlessness, since the EOs don't believe that sin is passed down biologically.
The EO article expressed other viewpoints as well, and the conclusion was that different views in Orthodoxy can be found on the topic.
The same phenomenon, a range of views appeared in the Russian article on baptism in heterodox churches. The question that the article was trying to answer was: Why is it that heterodox Trinitarians don't require rebaptism to convert to Orthodoxy? One view in the article expressed by theologians was that Catholic baptism as a sacrament is valid, and that this is the reason why rebaptism isn't required. The source of the belief that they don't need rebaptism is the very age old EO and patristic teaching and practice that they don't. Unfortunately, the early Tradition did not pass down an explanation of why they don't need baptism.
The Russian article considered that another EO view is that they don't need rebaptism because of "Ekonomia". But the same article, IIRC, criticized this idea and endorsed or seem to endorse the idea that they didn't need rebaptism because their initial sacrament was valid.
One idea is that the situation is like the story when Jesus heard that someone who was not part of the apostles' group was healing (or baptising also?) in His name. Jesus said that it was Okay because the person wasn't against them. I guess you could argue back that someone in the RC Church is implicitly against the apostles because the EO church and the RC one have different teachings on some issues.
Maybe one of the article's ideas was that you can't treat RC baptism as fulfilling the sacrament by "ekonomia" - either it is valid or not, and if it's not valid, then you can't use ekonomia to say that we can use it as a component of the person's sacramental life.
I would have to go through the article again, but my own view is that if people aren't requiring rebaptism, then the implication is that their first baptism was valid. Chrismation is a separate sacrament. If the first baptism was invalid due to incompletion, then something would have to be done to that baptismal rite itself to make it valid or complete or just do a valid one instead.