He was very strict in his interpretation of religious laws (Shab. 147a, 157b);In general I dislike statements of this sort, because it is so subjective, and so easy to interpret and reinterpret the evidence. How does one define strict (that in itself is somewhat subjective), and how does one define very? What is very strict? He objects in certain cases to practices he observed. But perhaps those are instances in which the actual law was in accordance with what he said, and he was correcting a misperception. Or perhaps he was under the impression that the metziut was such-and-such while it was really otherwise. Perhaps he is citing the standard practice of Eretz Yisrael, where he came from, and which differed from Babylonian practice, and thus had ample opportunity to comment on what he saw as Babylonian permissiveness. (And perhaps the same would be true for a Babylonian in Eretz Yisrael.) And perhaps in other instances besides the two examples cited, he is actually more lenient.
Perhaps he really was very strict, and a thorough analysis of all the stories ab and state
Let us start by examining the cases brought as evidence of Ulla's strictness.
The first was Shabbat 147a. There are two incidents with Ulla there:
It is possible that the author read לא קפדינן מידי as "we are not particular about it," meaning that "we are not stringent about it" while others such as Ulla are. If that is so, then we can point out that it rather seems that the being particular is about the presence of the dew or dust on the garments, such that if one does not really care that the dew or dust is on it, then there is no violation of Shabbat to shake it off. (And this is how the setama clearly understands it.)אמר רב הונא המנער טליתו בשבת חייב חטאת ולא אמרן אלא בחדתי אבל בעתיקי לית לן בה ולא אמרן אלא באוכמי אבל בחיורי וסומקי לית לן בה והוא דקפיד עלייהו עולא איקלע לפומבדיתא חזא רבנן דקא מנפצי גלימייהו אמר קמחללין רבנן שבתא אמר להו רב יהודה נפוצי ליה באפיה אנן לא קפדינן מידי אביי הוה קאי קמיה דרב יוסף א"ל הב לי כומתאי [חזא דאיכא] טלא עליה הוה קמחסם למיתבה ליה א"ל נפוץ שדי אנן לא קפדינן מידיR. Huna said: If one shakes out his cloak on the Sabbath, he is liable to a sin-offering. Now, we said this only of new ones, but in the case of old ones we have nought against it; and this is said only of black ones, but in the case of white or red ones we have nought against it; [but in any case there is no culpability] unless he is particular about them.
'Ulla visited Pumbeditha. Seeing the scholars shaking their garments he observed, 'The scholars are desecrating the Sabbath.' Said Rab Judah to them, 'Shake them in his presence, [for] we are not particular at all [about the clothes].' Abaye was standing before R. Joseph. Said he to him, 'Give me my hat.' Seeing some dew upon it he hesitated to give it to him. 'Shake it and throw it off,' he directed, '[for] we are not particular at all.'
More likely, the author saw that Ulla prohibited something while Rav Yehuda, the head of the academy of Pumpedita, told them that it was allowed because the action was not done to remove something about whose presence they really had concern.
It is unclear how Ulla would respond, for we do not have his response. It would seem that he is echoing Rav Huna that one who shakes out his cloak on Shabbat is liable to a sin-offering. Perhaps Ulla did not hold by this exception that it is only where one is makpid on the dirt. Or, quite possibly, he was under the cultural impression that they did this because they were truly concerned about the dirt on it. In the actual situation, now that they are not concerned, perhaps even Ulla would now say it is permitted.
Similarly, would we say that Abaye is being very strict in the parallel case brought down? Rather, both Rav Yosef and Rav Yehuda are teaching that in this case, we are not particular about the dirt or dew's presence, and in such a situation, Rav Huna's statement does not apply.
It is also possible (though much less likely) that the later story on the same page was intended:
'Ulla visited the academy of Assi b. Hini [and] was asked: Is it permitted to make a marzeb on the Sabbath? Said he to them, Thus did R. Ilai say: It is forbidden to make a marzeb on the Sabbath. What is a marzeb? — Said R. Zera: The capes worn by Babylonian women. R. Jeremiah was sitting before R. Zera [and] asked him, How is it thus? It is forbidden, replied he. And how is it thus? It is forbidden, replied he.R. Papa said: Adopt this general rule: Whatever [is done] with the intention of gathering it [the skirts] up is forbidden; whatever is for adornment is permitted. Just as R. Shisha son of R. Idi used to adorn himself with his cloak.But this is just answering an halachic question (where they may well have assumed that he would in other situations have said it was permitted, for otherwise, why ask?). He responds not based on his own thoughts of the matter, but rather by citing a tradition of Rabbi Ilai. And others immediately following also forbid certain things. That is why I think the first story was intended.
The second story they cite is from Shabbat 157b (the very end of the masechta):
AND FROM THEIR WORDS WE LEARN THAT WE MAY STOP UP [A SKYLIGHT] AND MEASURE AND TIE ON THE SABBATH. 'Ulla visited the home of the Resh Galutha and saw Rabbah b. R. Huna Sitting in a bath-tub of water and measuring it. Said he to him: Say that the Rabbis spoke thus of measuring in connection with a precept; did they rule [thus] when it is not in connection with a precept? — I was merely occupying myself, he replied.Thus, while the Mishna permitted measuring on Shabbat, Ulla held that this was just for the purposes of a mitzva, such as mikveh. However, measure for another purpose, like a bath, would be forbidden. Is this being strict? Well, Rabba bar Rav Huna agrees with him. His rejoinder is that he did not intend to measure by doing this, but was merely occupying himself. But real measuring would indeed be forbidden.
I see a trend in these two stories. In both cases Ulla made his statement based on an assumption of the internal state and intention of others, and in both cases, where the internal state is in fact different, Ulla's statement does not apply.
Though I've responded to these particular two cases, this does not mean that there is not a regular pattern of stringency. I have not made a scientific study of Ulla, to be able to make such a statement. Perhaps other incidents involving Ulla allow one to draw this general picture accurately.
Still, I would point out two instances in which Ulla does not take the stringent position, based on a very quick survey.
The first example I offer is in Shabbat 29b:
The synagogue overseer of Bazrah dragged a bench in front of R. Jeremiah Rabbah. Said he to him, in accordance with whom? [Presumably] R. Simeon! Assume that R. Simeon ruled [thus] in the case of larger ones, since it is impossible otherwise; did he say thus of small ones? Now, he disagrees with 'Ulla, who said: They differ [only] in respect of small ones, but as for large, all agree that it is permitted.Thus, according to Rabbi Yirmiyah Rabba, large benches are a dispute in which perhaps one may rely on Rabbi Shimon, but small ones are entirely forbidden. Meanwhile, Ullah would permit small ones according to everyone, and even large ones according to Rabbi Shimon. So here is a case where he is more lenient "in his interpretation of religious laws."
Another example, though it is unclear whether we would say he is being strict or lenient. Shabbat 13a:
'Ulla, on his return from the college, used to kiss his sisters on their bosoms; others say, on their hands. But he is self-contradictory, for 'Ulla said, Even any form of intimacy is forbidden, because we say, 'Take a circuitous route, O nazirite, but do not approach the vineyard.'Here, even though he apparently has a statement one way, in his own practical personal conduct he acted leniently. What do we make of this?
This is part of the reason I am reluctant to make such generalizations about personalities and approaches of Tanaaim and Amoraim.